Placenta, Cord, Fluid Presentation PDF

The Placenta and Umbilical Cord Normal Placenta  Accurate assessment of the placenta and umbilical cord is an important part of routine obstetric sonographic examination. Placental evaluation usually includes assessment of: Size Shape Consistency location The ev...

The Placenta and Umbilical Cord Normal Placenta  Accurate assessment of the placenta and umbilical cord is an important part of routine obstetric sonographic examination. Placental evaluation usually includes assessment of: Size Shape Consistency location The evaluation of normal umbilical cord by ultrasound would include: confirming the number of vessels documenting length appearance of the cord visualizing the points of insertion.  The Placenta Discoid organ that weighs 500-600 grams at delivery and is usually 2-4 cm AP in dimension The maternal surface is irregular and divided into cotyledons  Why do we care about the placenta? Placental pathology and/or abnormal placental development can result in a variety of complications to the pregnancy. The sonographer has the tools to perform a detailed evaluation of placental morphology, placental localization, and feto placental hemodynamics.  Placental pathology is often nonspecific, findings lead the healthcare team toward further investigation to minimize risks to fetal outcomes. Structural lesions of the placenta can result in vascular resistance changes, severely compromising the fetus. the morphology and vascular condition of the placenta and umbilical cord may suggest the presence of coincident fetal anomalies. umbilical cord compression, placental tumor, or lesions due to maternal diseases such as hypertension or diabetes can compromise the placenta and fetus  Development of the Placenta The placenta is composed of both a maternal portion (arising from the endometrium) and a fetal portion (arising from a section of the chorionic sac). The functional layer of the endometrium in the gravid woman is called the decidua. the decidua basalis is the deeper portion and develops into the maternal side of the placenta the decidua capsularis overlies the embryo the decidua parietalis/vera encompasses all of the remaining decidua.  The Placenta Attachment to the uterine wall can occur anywhere within the uterine cavity. At the fetal side is a fused layer of amnion and chorion (chorionic plate) with underlying fetal vessels. At the maternal side are about 20 functional lobes, or cotyledons, composed of maternal sinusoids and chorionic villous structures Sonographic appearance: The placenta is a relatively homogeneous organ that may exhibit varying degrees of calcification and anechoic spaces (lacunae) in later pregnancy.  Functions of the Placenta Functions of the placenta: Conversion of fetal steroids to estrogen Secretion of progesterone Secretion of hCG Exchange of oxygen, waste products, & nutrients between the fetus and the mother  Placental Circulation The maternal and fetal circulations are separate Oxygenated maternal blood is pumped through spiral arterioles within the decidua basalis and enters the intervillous spaces (sinus) surrounding and bathing the villi. Gases and nutrients exchange across the walls of the villi, allowing nutrition, respiration, and waste removal to take place. The resulting oxygenated blood within the villous capillaries returns to the fetus through the umbilical vein. Deoxygenated fetal blood, carried to the placenta by the umbilical arteries, circulates through capillaries in the chorionic villi within the placental lobes.  Placental Circulation During pregnancy, maternal blood volumes increase to satisfy fetal needs. The many vascular channels and sinusoidal structure of the placenta create a low-impedance system (low resistance), even lower than other fetal vascular beds, at least until late in pregnancy.  Doppler investigation of maternal and fetal vessels demonstrates the low-resistance nature of the placenta. Anything that causes increased placental resistance or placental insufficiency can have profound effects on the developing fetus. Sonography of the Placenta By the end of the fourth month, sonography helps identify the final shape of the placenta. It is usually discoid. The normal cord insertion is approximately in the central portion, but the cord may insert eccentrically near the margins (battledore placenta) or below the edge of the placenta (velamentous insertion) In general, the wider the base of attachment, the thinner the placenta. Evaluation of placental length, volume, and thickness allows for pregnancy outcome correlation. In general, a placental thickness of greater than 4 cm prior to 24 weeks’ gestation is considered abnormal  Sonography of the Placenta Transabdominal scanning with a curvilinear transducer is generally accepted for evaluation At times, linear transducers can be used to evaluate the anterior placenta in thin patients. Endovaginal transducers allow visualization of the placenta and umbilical cord earlier than with transabdominal imaging. The proximity to the os, and an empty bladder, make the endovaginal approach ideal for evaluating questionable previas late in pregnancy. Transperineal ultrasound is a valuable when endovaginal imaging is contraindicated or not available, particularly in evaluating the relationship between the placental edge and the internal os of the cervix. Often in late second- or third-trimester pregnancies, the endocervical canal is obscured transabdominally, because of obstructive fetal parts and the inability to maintain a full bladder. With transperineal ultrasound, this area can be clearly imaged in most cases.  A Full Bladder A properly full bladder enhances placental visualization in early pregnancy and improves imaging of the lower uterine segment later in gestation.  Especially important when a diagnosis of placenta previa is being considered. An empty bladder may preclude visualization of the cervical os, but an overdistended bladder can cause a false-positive appearance of placenta previa.  A very full bladder causes the anterior and posterior walls of the lower uterine segment to be very close together, producing what appears to be the internal cervical os at a falsely superior location. The sonographer can improve patient comfort and reduce the possibility of false- positive images of placenta previa by varying the degree of bladder distention during the examination (i.e., start with full bladder, then have the patient empty 1 cup of urine at a time). In general, a bladder is considered adequately full when cervical length is between 3 and 5 cm.  To relieve severe discomfort in late second- and third-trimester pregnancies, once the cervical area has been evaluated, the patient may void before the remainder of the examination is performed. Curved linear image of an anterior placenta showing the distinct chorionic plate (arrow). Posterior placenta demonstrating the hypoechoic retroplacental zone  Scanning Protocol Placental evaluation should be part of every obstetric scan Include documentation of the placental location in relation to the internal cervical os. In the case of an accessory (succenturiate) lobe, note the location along with the position of the connecting tissue or membranes.  Evaluate the structural texture, morphology, and any abnormalities of the placenta and the insertion site of the cord. Placental thickness should be determined. If there has been any vaginal bleeding or abdominal pain, examine the retroplacental area for possible areas of elevation which could signify abruption or hemorrhage.  Measurement of Placental Thickness The sonographer should first identify the placenta myometrial interface in an unobstructed midpoint position. The measurement should exclude the myometrium and retroplacental complex. Ideally, the transducer should be perpendicular to the placenta lest the resultant measurement be falsely high. Evaluation of this measurement should take into consideration the shape of the placenta. For example, a broad-based placenta may be thinner and a narrow-based placenta thicker, without indicating the presence of any lesion.  Placental Texture Placental texture changes from an echogenic focal thickening of the wall of the gestational sac early in pregnancy to the fine, granular, homogeneous texture seen from the end of the first trimester. In the second and third trimesters, intraplacental and subchorionic vascular spaces may sometimes be seen and should be assessed carefully, but are not usually significant clinically Areas of varied echogenicity should be carefully documented. Document multiple cystic or vesicular appearance; irregular, hypoechoic areas; or highly echogenic areas with hypoechoic margins There is an association between increased levels of AFP and the findings of large vascular spaces. A late third-trimester placenta may exhibit cystic areas located centrally within clearly delineated lobes. These nonvascular areas may represent areas of necrosis. (not to be confused with placental lakes)  The Retroplacental Complex This zone is composed of the decidua basalis and portions of the myometrium, including maternal veins draining the placenta. Identifiable as early as 14 weeks hypoechoic area 10 to 20 mm deep to the placenta. Proper identification of this area is important. A prominent anterior RPC can lead to excessive bleeding during invasive procedures such as amniocentesis or cesarean section. In addition, the RPC can mimic abruption, degenerating fibroids, or hydatidiform mole. Real-time visualization of blood flow in this area helps distinguish the normal RPC from the aforementioned pathologic conditions. Large venous channels may be visualized within the complex, most commonly in posteriorly located placentas, where the effects of gravity-induced pressure can overdistend the veins. Color Doppler image of the retroplacental vessels  Placental Grading As the placenta ages through the gestation, structural changes occur. A method of “grading” the placenta was devised to help determine gestational age and was once thought to help predict fetal lung maturity. Statistical correlation between gestational age and placental grading is actually very poor. The identification of a grade II placenta in the second trimester or early third trimester can suggest placental insufficiency, especially if there are underlying maternal complications.  Grade 0 Placenta Should be seen 28-31 weeks No calcifications, smooth chorionic surface  Grade 1 Placenta  Should be seen 31-36 weeks  Scattered calcifications  Slight contouring of chorionic surface  Grade 2 Placenta Should be seen 36-38 weeks Basal layer calcifications can be seen Grade 3 Placenta Should be seen no earlier than 38 weeks + Basilar calcficiations Interlobar septal calcficiations infarcts The Abnormal Placenta  Placental Variants Extrachorial types Placenta in which the membranous chorion does not extend to the edge. 20% of delivered placentas show partial regions like this can be associated with antenatal or postpartum hemorrhage Two types: Circumvallate Circummarginate  Circumvallate Circumvallate fetal membranes (chorion and amnion) "double back" on the fetal side around the edge of the placenta. Small central chorionic ring surrounded by thickened amnion and chorion May be predisposed to separate from the uterine wall early May be predisposed to antepartum bleeding and threatened AB After delivery, a circumvallate placenta has a thick ring of membranes on its fetal surface  Circumvallate placenta. Rolled peripheral edge of the membranes as they insert at some distance in from the placental edge. Fibrin Deposition  Pooling of maternal blood in the subchorionic space.  Ultrasound image of a circumvallate placenta demonstrating the characteristic rolled-up placental edge. Ultrasound image of a circumvallate placenta appearing as a linear structure in the amniotic cavity. This can be misinterpreted as a uterine synechia Circummarginate Central attachment of the membranes without a central ring. Circummarginate placenta. Flat transition of the membranes inserting at some distance in from the placental margin. Accessory Types of Placentas Due to variation in how the placenta forms, three variants can occur: Succenturiate – accessory cotyledon with vascular connections to the main placenta Bipartite – placenta is divided into two lobes, but united by primary vessels and membranes Annular – ring shaped placenta Succenturiate Placenta Succenturiate  Ultrasound image demonstrating two placental masses (arrows) consistent with a succenturiate lobe of the placenta. Bipartite/Bilobed Bipartite/Bilobed Annular Placenta  Abnormal Placental Thickness Normally the placenta is less than 5 mm AP Increased placental thickness (placentomegaly) is defined as AP measurement over 5 cm and has multiple causes: Gestational diabetes appears that way due to Rh isoimmunization retroplacental clot) Maternal infection Chorioangioma Multiple gestation Maternal anemia Hydrops fetalis Sacrococcygeal teratoma Partial mole Chromosomal abnormalities Abruption (not really thick, just Thickened Placenta (be careful with fundal placentas !) Placentomegaly Fundal Placenta  Decreased Placental Thickness Measures less than 1.5 cm AP Causes: Pre-eclampsia IUGR Diabetes mellitus (pre-dating pregnancy) Intrauterine infection Caution with polyhydramnios – it can make it appear thinner!  Abnormal Placental Location - Placenta Previa Placental tissue encroaching on the cervix and or crossing the internal cervical os. Caused by abnormally low implantation of the blastocyst. More common in multiparous women, previous C-section patients, or patients with multiple abortions. The CLASSIC symptom is painless vaginal bleeding during the third trimester. Best diagnosed sonographically in the third trimester due to placental migration. Also, an over distended maternal bladder or focal myometrial contraction can lead to a false diagnosis of previa.  Classification of Previa Complete/total/central previa Placenta completely covers internal os of the cervix. May be symmetric or asymmetric. Partial previa Partially covers os Marginal previa Encroaching, but not crossing the os Low lying placenta Not a type of previa technically Placenta is within LUS within 2 cm of the os. Previa Previa – endovaginal image  Placental Abruption Premature separation of all or part of a normally implanted placenta from the myometrium. Predisposing factors: Maternal hypertension Advanced maternal age Multiparity Maternal vascular disease Cigarette smoking Uterine leiomyoma Symptoms: Abdominal pain May or may not have vaginal bleeding  Placental Abruption– two types Concealed 20% of cases Hemorrhage is confined to the uterine cavity Detachment may be complete and the consequences are severe. May be diagnosed sonographically External Blood drains through the cervical os Patient presents with PAINFUL vaginal bleeding. Detachment is usually not as severe. Sonographic diagnosis can be difficult.  Abruption: Sonographic Findings Elevation of the placenta from the uterine wall Retroplacental fluid collection of varying echogenicity (depending on age of blood – most likely hypoechoic) Placenta may appear normal Placenta may appear thickened (due to retroplacental hemorrhage)  Sagittal ultrasound image with partial separation of the placenta from the uterine wall. Note the raised placental edge (arrow) wit h a large accumulation of blood adjacent to the cervix (CX).  Abnormal Placental Adherence Uncommon condition resulting from defective decidual formation Causes an abnormal attachment to the uterine wall Predisposing factor – uterine scarring Villi may grow into a C-section scar. Associated with placenta previa. Hysterectomy may be necessary due to risk of hemorrhage  Three types of abnormal adherence Placenta Accreta Chorionic villi are in direct contact with the myometrium, but do not invade Placenta Increta Chorionic villi invade the myometrium Placenta Percreta Chorionic villi penetrate and perforate the myometrium  Sonographic Findings Depends on the type of pathology present Loss of normal hypoechoic retroplacental vascular complex Local basal plate thinning (accreta) Increasing myometrial thinning (increta) Focal myometrial bulge (percreta) Sonographic diagnosis is difficult irregular interface between the myometrium and the maternal bladder  Intraplacental Lesions Placental Lakes The presence of large pools of maternal venous blood in the placenta. Lakes are of no consequence clinically and appear as an anechoic/hypoehoic rounded area in the placenta. May exhibit very slow venous flow  Fibrin Fibrin Deposition Pooling of maternal blood in the subchorionic space. Clinically insignificant Hypoechoic material beneath the chorionic surface of the placenta Intervillous thrombosis Caused by fetal bleeding into the intervillous space Increased incidence with Rh factor incompatibility  More Intraplacental Lesions Placental Infarcts Ischemic necrosis of placental villi occurs as a result of obstruction of the spiral arteries and is usually found at the periphery of the placenta If circulation is normal, there are rarely complications Occurs most commonly in eclampsia, pre-eclampsia May also see intervillous thrombus. may be associated with retroplacental hemorrhage in up to 25% of term placentas. may be associated with IUGR The diagnosis is often pathologic and there is no specific sonographic appearance: Anechoic or hypoechoic areas seen in placenta May be small or large Absence of blood flow on color or spectral Doppler Chorioangioma Chorioangioma Vascular tumor of the placental tissue 1 in 5,000 pregnancies Complications may arise when the tumor is large (Greater than 5 cm) Polyhydramnios Fetal hydrops Associated with increased MSAFP Sonographic Findings: Solid, well circumscribed placental mass, possibly near cord insertion site Chorioangioma Umbilical Cord Development The umbilical cord originates from fusion of the yolk sac stalk and the omphalomesenteric duct at approximately 7 weeks gestation. The urachus, an outpouching from the urinary bladder, forms the allantois and thus the definitive umbilical vessels. Structure and Function The umbilical cord normally contains two arteries and one vein surrounded by a mucoid connective tissue (Wharton’s jelly), all enclosed in a layer of amnion. The vein brings fresh, oxygenated blood to the fetus and the arteries carry deoxygenated blood from the fetus to the placenta.  The umbilical arteries are longer than the vein and wind around it. The vessels are longer than the cord itself, resulting in twisting and bending of the cord and vessels The cross-sectional area of the cord has been found to correlate with gestational age, up until it reaches a plateau at 32 weeks.  At term, the average length of the cord is about 51.5 to 61 cm, with a mean circumference of 3.8 cm.  Scanning Technique The umbilical cord is visualized best in the late second and early third trimester, when amniotic fluid volume is at its peak. Because of the extreme length of the cord, it is difficult, on a routine basis, to scan it in its entirety to rule out abnormalities. If a lesion is suspected, every effort must be made to view as much of the cord as possible. Changing the mother’s position from side to side may help shift the position of the fetus in relation to the cord.  The placental insertion site of the cord, as well as its entry site to the fetus (to rule out, for example, an omphalocele) should be imaged and documented routinely. Document the presence of the three vessels within the cord. Visualization of the number of cord vessels is difficult when there is oligohydramnios may be troublesome in the third trimester even in a normal fetus because of the relative lack of fluid and the presence of potentially obstructing fetal parts. Any abnormality in cord appearance, size, position, degree of coiling, or attachment sites to placenta or fetus should be noted.  Normal Sonographic Appearance In early pregnancy, the umbilical cord may appear as a series of short, linear echoes extending from the fetus to the placenta. As pregnancy progresses, transverse images through the cord reveal the three circles representing the larger vein and two smaller arteries. Longitudinal images at this stage reveal a series of parallel linear echoes within the amniotic fluid that may exhibit the characteristic twisted appearance of the cord. The “stack of coins” appearance refers to visualization of several portions of cord folded on each other. Arterial pulsations may be demonstrated within the umbilical arteries. The insertion of the umbilical cord into the placenta should be Abnormal Umbilical Cord  Single Umbilical Artery Single Umbilical Artery MOST COMMON abnormality By itself, not a concern, but it may be associated with other anomalies. Genitourinary abnormalities Trisomies 18 and 13 Cardiovascular anomalies Central Nervous system anomalies Omphalocele Sonographic findings: One umbilical artery May be larger Two vessel cord in transverse section Fully evaluate fetal anatomy SUA  Umbilical Cord Cysts Developmental and asymptomatic Focal accumulations of Wharton’s Jelly Can appear as cysts Originate as a remnant of the omphalomesenteric duct (cyst close to fetus) or the allantoic duct (cyst far away from fetus) Sonographic findings: Umbilical cord cyst Lack of flow in cyst  Nuchal Cord Wrapping of the cord around the neck Presents in 20% of pregnancies Rarely associated with complications Color Doppler can be useful to demonstrate a nuchal cord Care must be taken to differentiate cord draping across the back of the neck vs. cord wrapped AROUND the neck. How could you demonstrate this? Cord Prolapse Cord protrudes through the cervix or adjacent to fetal part that is presenting. This is an emergent situation  Vasa Previa Fetal vessels cross the cervical os. Pass between the cervix and the presenting fetal part Membranes are intact Associated with velamentous insertion Vasa Previa  Umbilical Vein Thrombosis Torsion, knotting, or compression of the cord may cause stasis and thrombosis. Thrombosis may also occur after intrauterine transfusion or during fetal blood sampling. More frequently seen in infants with diabetic mothers or fetuses with non-immune hydrops Fetal death almost always occurs Sonographic findings: Increased echogenicity in the lumen of umbilical vessels Absence of color and spectral Doppler signals within umbilical vessels Umbilical Cord Knots Rare, associated with mono, mono twins Sonographic appearance: Multiple loops of cord seen in one scan plane  Placental Cord Insertion Sites Normally, the cord inserts centrally into the placenta Marginal/battledore insertion: Attachment of the cord at the periphery of the placenta Cord enters directly into the edge of the placental tissue Velamentous Cord Insertion Attachment of the cord to the membrane vs. the placental mass The cord travels beneath the chorion for some distance before it attaches to the edge May be associated with IUGR, preterm birth and congenital anomalies Sonographic findings:  Establish relationship between cord insertion and placental mass.  Color Doppler may assist in diagnosis. Marginal Insertion A: Central insertion of cord into placenta. B: Battledore insertion. Cord is inserted near the margin or edge of the placenta. C: Velamentous insertion. Cord is inserted into chorioamniotic membranes, which extend beyond the placental parenchyma and lie along the uterine wall. Location of this type of insertion near the lower uterine segment can lead to complications such as vasa previa. Amniotic Fluid Physiology and Production Produced by the fetal kidneys, tissues, skin, and membranes Amniotic fluid is removed from the fetus by the GI tract, lungs, membranes and cord Functions: Cushion Pressure equalization Temperature regulation Prevents adherence to membranes Reservoir for fetal metabolites Essential for the development of the lungs Sonographically – anechoic, but may have echogenic foci/debris (vernix, cellular debris) Fluid Estimation - AFI We use AFI to estimate the amount of fluid. Technique: Divide the uterus into four quadrants using the linea nigra as the vertical axis and the umbilicus as the horizontal axis The pocket with the largest vertical unobstructed dimension is measured in each quadrant. Sum of all four measurements = AFI Keep the transducer perpendicular to the bed AFI should increase until about 28 weeks, then it will slowly decrease As a guideline: Normal AFI = 5-22 cm  25cm = polyhydramnios  Fluid Estimation - Single Deepest Pocket oMeasure the dimensions of the largest vertical pocket of amniotic fluid. oManning et.al.1981. : Pocket of fluid 8cm = polyhydramnios  Single Deepest Pocket vs. AFI Measuring amniotic fluid volume remains an important part of fetal assessment. The SAFE trial (Ultrasound Obstet Gynecol, 2016), a multicentered randomized controlled trial (RCT), was designed to answer the question as to whether AFI or SDP technique is better in predicting pregnancy outcome. 1052 pregnant women at term with singleton pregnancies were included in this trial which included low risk as well as high risk indications, such as gestational diabetes (GDM), hypertensive disorder, fetal growth restriction, placental insufficiency or intrahepatic cholestasis of pregnancy. KEY POINTS: AFI was associated with more women being identified with oligohydramnios, but without any significant benefit in perinatal outcome The SDP appears to be the favorable approach when estimating amniotic fluid volume in both high and low risk patients Some labs do a single vertical pocket prior to 22 weeks and an AFI after that.  Oligohydramnios Abnormally decreased amount of amniotic fluid Associated with: (DRIPP) Demise Renal Abnormalities IUGR Post dates PROM Also can be associated with eclampsia, preeclampsia Anhydramnios Complete absence of amniotic fluid  Polyhydramnios Abnormally increased amount of amniotic fluid May be acute or chronic Causes: Single Deepest Pocket Maternal Gestational diabetes measurements: Rh incompatibility Mild – vertical pocket 8-12 cm Fetal Causes Moderate – vertical pocket 12- CNS anomalies 16 cm GI anomalies Severe – vertical pocket > 16 Facial clefts, masses cm Fetal hydrops Twin-twin transfusion syndrome Sacrococcygeal teratoma Skeletal dysplasia Amniocentesis Method of retrieval of amniotic fluid for analysis Rate of complications is 0.5% Genetic amniocentesis can be performed to diagnose chromosomal conditions around 16 weeks Late 2nd and 3rd trimesters, can be performed to assess fetal lung maturity through two tests: Lecithin/sphingomyelin ratio (LS ratio) A ratio of these two greater than 2:1 indicates respiratory distress syndrome would be unlikely.  Most accurate method Phosphatidylglycerol (PG) Appears at the time of lung maturity so it can be detected in the fluid FETAL MEMBRANE ABNORMALITIES  Membranes Various intrauterine membranes, septations, and bands have been demonstrated with sonography in and about the amniotic cavity. The most commonly identified structures or conditions include chorioamniotic separation elevation due to subchorionic hemorrhage membranes associated with multiple gestations and blighted ova intrauterine synechiae Recognition of these common benign types of membranes is important to ensure that they are not confused with amniotic bands.  Chorioamniotic Separation The placental (chorion) and fetal (amnion) membranes are separate early in gestation and fuse together at about the 14th week of gestation. In rare instances, chorioamniotic separation can occur later in gestation. This can be focal or extensive and is usually the result of intervention such as amniocentesis, although it may be sporadic. Sporadic cases have been associated with both chromosomal and developmental abnormalities of the fetus. Sonographic diagnosis is possible when the amnion is visible as a discrete, free-floating membrane separate from the chorion surrounding the fetus.  Intrauterine Synechiae (see-neek-e-ay) Amniotic sheets or intrauterine synechiae are linear, extraamniotic tissues that project into the amniotic cavity. These are usually an incidental finding and occur in approximately 1 in 200 pregnancies and in up to 15% to 49% of women who have had uterine curettage performed Although they appear to be within the amniotic fluid, they are anatomically external to the amniotic sac. On ultrasound, they appear as linear protrusions continuous with and of the same echogenicity as the placenta. Synechiae do not cause fetal entrapment, or adhere to the fetus, and are not associated with fetal anomalies. Typically, they are no longer visible in the late third trimester secondary to obliteration or compression.  Differential diagnoses should include a uterine septum, subchorionic hemorrhage, circumvallate placenta, and a multiple gestation with absence of a fetus in one sac.  Amniotic Band Syndrome Sporadic condition that is thought to occur as a result of rupture of the amnion without rupture of the chorion This leads to transient oligohydramnios and passage of the fetus from the amniotic to the chorionic cavity. Early rupture can lead to severe malformations of the cranium, central nervous system, face, and viscera. Amniotic bands may tear or disrupt previously normally developed structures, leading to congenital amputations, constriction rings, and bizarre nonanatomic facial clefts. Amniotic band syndrome may be detected sonographically by demonstrating fetal malformations in a nonembryonic distribution and by direct visualizations of the amniotic bands