First Trimester Ultrasound: Normal Anatomy

First Trimester Normal Exam

• Ultrasound alone is more accurate than using a menstrual date when determining gestational age in the first or second trimester, specifically up to 23 weeks in spontaneous conceptions.
• Crown-rump length (CRL) is the best parameter for assessing gestational age.
• If more than one first-trimester scan exists, use the earliest MSD or CRL measurement.
• The earliest CRL should be equivalent to at least 7 weeks (or 10 mm) to determine gestational age.
• Three CRL measurements are necessary.
• CRL is most precise if it is less than or equal to 84.0mm.

Fetal Head - Normal Anatomy in 1st Trimester

• Cranial bones should be ossified.
• There must be no defects in the skull bone.
• Lateral ventricles are filled with echogenic choroid plexus.
• The falx, dividing the two sides of the brain, should be symmetrical.
• CP represents choroid plexus.
• Lat. V denotes the lateral ventricle.
• Thal. stands for the thalamus.
• Cer. Ped. represents the cerebral peduncles.
• AS denotes the aqueduct of Sylvius.
• T equals thalamus.
• M stands for midbrain.
• B equals brainstem.
• MO stands for medulla oblongata.
• NT equals nuchal translucency.
• Note the nasal bone, palate and mandible.
• Maxilla C equals maxilla gap.
• Ensure the stomach is seen on the left side of the fetus.
• The 4-chamber heart points to the left side of the fetus.
• Fetal spine looks like two little railroad tracks, or tiny vertebrae.

Fetal Abdomen Anatomy

• No assessment of genitalia at this stage.
• Fetal kidneys appear echogenic.
• A small fetal bladder is identified; it will appear snichoic (fluid filled).
• Be cautious when using color Doppler.
• The two arms and two legs can be visualized.
• Check for two hands and two feet.
• Assess the orientation of limbs.
• Ensure the bones are ossified.
• Assess for the presence of each bony segment of each limb (femur, tib-fib, humerus, ulna-radius).

Omphalocele

• 12 weeks/CRL > 45mm

• Midline, covered by membrane
• Size inversely correlates with chromosomal abnormalities
• 20-40% associated anomalies
• Aneuploidy (T18), BWS, structural (cardiac)

Gastroschisis

• Para-umbilical defect, right of UC
• Herniated free floating bowel loops without membrane
• Rarely associated with chromosomal or structural abnormalities

1st Trimester Detection of Structural Abnormalities

• Major defects in euploid fetuses ~2%

Transabdominal vs Transvaginal Sonography

• Patient Preparation includes distended urinary bladder for Transabdominal (TA) and empty urinary bladder for Transvaginal (TV)
• Transabdominal: A distended urinary bladder displaces bowel and provides a good acoustic window
• Drink one liter of clear fluid without voiding or use a foley catheter or intravenous drip for a full bladder.
• Overdistended bladder: Have the patient partially void and rescan
• Overdistended can cause extreme patient discomfort, compression may distort anatomy and structures may be pushed out of field of view
• Overdistended may obscure an early gestational sac and may artificially elongate the cervix (placenta previa)

Obstetrical Patient History

• Indication for the exam includes why the exam is being done like routine screening, dates, problems, maternal disease or presented symptoms.
• Specific info includes the age of pregnant patient, gravidity, parity and LMP.
• History includes previous pathology, family hx of anomalies, previous surgeries.
• Diagnostic Tests: Maternal Serum Testing, amniocentesis and Chorionic Villous sampling.

Goals of First Trimester Sonography

• Visualize and locate the gestational sac
• Locate the sac as intrauterine, ectopic (extrauterine), or both
• Identify embryonic demise and anembryonic (no embryo) gestations
• Make a determination about the number of embryos
• multiple embryos = Chorionicity (how many chorions?) and Amnionicity (how many amnions?)
• Estimate the menstrual age of the pregnancy
• Diagnose fetal abnormalities early

Weeks 1-2 of Pregnancy

• Days 1-14 of the mother's menstrual cycle
• Menstrual phase (days 1-5): primordial ovarian follicles and thin endometrium
• Follicular/Proliferative phase (days 6-13): dominant Graffian follicle reaches 2 cm and endometrium begins thin, grows to 4-8 mm
• Ovulation (day 14): ovum is released and trilaminar endometrium

Weeks 3-4 of Pregnancy

• Fertilization happens in the ampulla of the fallopian tube
• The conceptus enters the uterus (zygote to a morula to a blastocyst)
• The blastocyst implants into the endometrium
• Primary yolk sac forms, gets pinched off and forms secondary yolk sac
• The placenta begins to develop

Normal Intrauterine Gestation

• Intradecidual sign refers to the earliest possible demonstration of a gestational sac by ultrasound between 4 ½ – 5 weeks (Transvaginal)
• Appears as a small gestational sac within the decidua (Rumack, Fig 31-6) with No fetal pole or yolk sac seen
• Gestational sac looks empty as it is Too early to see embryo or yolk sac
• Double Decidual Sign refers to sonographic visualization of the 3 layers of the decidua in early pregnancy (Decidua basalis, Decidua capsularis, Decidua parietalis) described in Rumack Figure 31.5
• Formed by the reflection of decidua parietalis and decidua capsularis with the endometrial cavity in between
• Requires visualization of 2 echogenic layers
• Ring 1 stands for Echogenic decidua capsularis (inner) and Ring 2 = Echogenic decidua parietalis (outer layer)

Sonographic Appearances of a Normal Intrauterine Gestation

• Gestational Sac can be seen at 5 weeks TA (Mean Sac Diameter ≈ 5mm) or TV = 4 1/2 weeks (Mean Sac Diameter ≈ 2-3mm)
• Gestational Sac grows ≈ 1mm per day and Round Sac initially becomes oval as it enlarges
• Gestational Sac implants immediately beneath the echogenic endometrial stripe and initially filled with chorionic fluid
• First structure to be seen normally in the gestational sac at approx 5 1/2 weeks.
• The upper limit of normal is 5.6 mm (internal diameter.)
• Remains connected to the embryo by the vitelline duct (yolk stalk) which is part of the umbilical cord
• Record number of YS's (# of YS's usually corresponds to # of amnions)

Double Bleb Sign

• The earliest demonstration of the amnion (5 ½ weeks), identified by 2 blebs or bubbles such as Amnionic cavity + Yolk sac
• An Embryonic disc (more than 2mm) is situated between the 2 blebs
• The Amnion grows to meet the chorion by 12 weeks and fuses at 16 weeks.

Early Embryo and Cardiac Activity

• Cardiac activity will routinely be seen at 5 weeks and a CRL of 5 mm
• It is Documented by an M-mode tracing (or video clip), as shown in Rumack Figure 31.9
• If the embryo is smaller than 5 mm, then may need to bring patient back to record heartbeat

Umbilical Cord

• By the end of the 6th week, the amnion expands and envelops the connecting stalk, yolk stalk, and the allantois to form the umbilical cord
• Do not apply Doppler to embryo unless necessary (ALARA)

Contains

• 2 umbilical arteries -become the medial umbilical ligaments.
• 1 umbilical vein -becomes the ligamentum teres.
• Allantois -becomes the urachus and median umbilical ligament.
• Yolk stalk (vitelline duct) - connects primitive gut to the yolk sac

Week 6 of Intracranial Cystic Structures

• 3 primary brain vesicles form

• Prosencephalon (forebrain)

• Mesencephalon (midbrain)

• Rhombencephalon (hindbrain)

• Between 6-8 weeks the normal intracranial cystic rhombencephalon can be seen in the posterior aspect of the embryonic head

Gestational Sac Size

• In weeks 5-10, the first structure seen before visualization of a YS or embryo is the GS.
• Pregnancy should be followed until an embryo with cardiac activity is seen.
• Accurate to within 1 week of MA
• Measurements are taken from inner to inner border
• gest sac grows 1mm per day,
• Mean Sac Diameter is calculated with MSD = L + W + H / 3

Crown Rump Length

• Weeks 6-13 (maybe seen at week 5 by TV) and is Accurate to within 5-7 days of menstrual age
• Measure the length of the embryo (or fetus) from the top of the head to the bottom of the torso
• Use the Longest of 3 measurements or the average of 3 measurements (note most machines calculate average)

Crown Rump Length Measurements

• NEED to take 3 CRL measurements!
• Becomes less accurate towards the end of the 1st trimester due to fetal flexion and extension
• The measurement must be taken in a midsagittal plane and must be not oblique

Topic 9 - Multiple Gestation

• Older first-time mother in cad mist likely to have twins also increase with age and this is because they have higher levels of naturally occurring follicle- stimulating hormone, (stimulates ovulation an increases the possibility of multiple eggs being released.

Multifetal Pregnancies

• Are the most common, accounting for ~1% of all pregnancies, seen more with the use of in vitro fertilization (IVF)
• A twin pregnancy can be broadly categorized into: 70% dizygotic and 30% monozygotic

Dizygotic vs Monozygotic Twins

• Dizygotic fraternal twins resulting from independent fertilization of two ova who always two chorions and two amnions
• This feature alone cannot be used to distinguish dizygotic from monozygotic pregnancies as approximately 20% of monozygotic pregnancies will also be dichorionic-diamniotic resulting in ~80% of all twin pregnancies being dichorionic-diamniotic).
• Monozygotic Also known as identical twins due to fertilization of a single ovum that then splits into two who unlike dizygotic pregnancy, do not have maternal age, family history, or ethnic predisposition. *the time at which this separation occur will vary.
• Amnioticity and chorionicity are determined by documentation of different sex fetuses
• The most reliable sign of dizygosity on ultrasound is documentation of different sex fetuses determine how many chorions and how many amnions are present.

Division at and Risk factors for dizygotic

• Morula Stage (1-4 days) = dichorionic-diamniotic (DCDA) (76% of all twin pregnancies and 20-30% of monozygotic twin pregnancies
• Blastocyst Stage (4-8 days) = monochorionic-diamniotic (MCDA) ( 30% of all twin pregnancies 70-75% of monozygotic twin pregnancies.
• Implanted Blastocyst (1-2 weeks) = monochorionic-monoamniotic (MCMA)1.5% of all twin pregnancies
• 1-5% of monozygotic twin pregnancies, Developed embryoblast (>2 weeks) = conjoined twins such as <0.3% of all twin pregnancies and <1% of monozygotic twin pregnancies
• Zygosity can either be are Monozygotic one zygote or Dizygotic and two zygotes
• Time of Division : Post-Conception determine the Dichorionic Diamniotic Twins occur Before Day4 , Day4 to Day8 -Monochorionic Diamniotic and After Day8 - Monochorionic Monoamniotic and As late as Day 13....Conjoined twins

Monozygotic vs Dichorionic Division

• Division between days 0-4 yields two chorions and two amnions (~26% of MZ twins)
• Between days 4-8 Division yields one chorion and two amnions (Monochroionic/Diamniotic – MC/DA ~70% of MZ twins)
• Between days 8-13 Division yields one chorion and one amnion so Monochorionic/monoamniotic MC/MA (~4% of MZ twins)
• Beyond 13 days, results in embryonic fission being incomplete and results in conjoined twins MC/MA (*RARE (1 in 50,000/100,000))

Sonographic Determination of Chorionicity and Amnionicity

• Up to 10 weeks gestation, count the number of gestational sacs each which forms its own placenta/chorion

• Always remember that the 2 gest. sacs = DC and one gest. sac = MC (gestational sac = chorionic sac/chorionic fluid)

• Count the # of embryos and embryonic heartbeats. If MC twins (one gest. sac) is detected, seek the presence of one or two amniotic sacs After the first trimester: Determine if there are Determine fetal sex

• Male/ female = dichorionic/diamniotic

• Same sex = not useful information in determining

• two separate placentae = dichorionic/diamniotic.

• Chorionic membrane in the uterus

• "twin peak (AKA Lambda sign) = DC/DA

• The chorionic T sign = MC/DA.

• The key is looking at the presence and thickness of the Inter-twin membrane between them

Twin Perinatal Loss

• Prematurity
• Intrauterine Growth Restriction (IUGR)
• Discordant growth means twins are not developing at the same rate.
• Monochorionic complications:
• Cord entanglement, etc.

1st Trimester Screening

• Includes Nuchal Translucency and Biochemical Testing

Nuchal Translucency

measurement

• Should occur between 11w0d to 13w6d,
• CRL measurements,
• midsag plane,
• only head/chest on image.

The Canadian Prenatal Screening Guidelines

• There are multiple prenatal (antnatal)aneuploidy screening options for pregnant patients.

• They identify the risks of chromosomal anomalies & other fetal abnormalities.

• These tests include: Ultrasound Blood tests Genetic testing (DNA testing) The Society of Obstetricians and Gynecologists in Canada (SOGC) has federal recommendations. Each province & territory is responsible to develop their own process.

Summary statements published in the Journal of Obstetrics and Gynaecology Canada:

• It is recommended that all pregnant patients be offered a 1st trimester Ultrasound, completed at 11 to 14 weeks gestation
• Ultrasound offers many advantages for prenatal screening and pregnancy management like accurate dating, determination of twin chorionicity, and early detection of some major structural abnormalities, regardless of the aneuploidy screening options.

Prenatal Invasive Testing Know

amniocentesis & chorionic villous sampling (CVS) 1st trimester amnio or CVS: lower risks (than what is reported in literature) 2nd trimester Amniocentesis: increased risk of fetal loss CVS Transcervical: higher risk of pregnancy loss CVS Transabdominal: lower risk of pregnancy loss Prenatal Screening for Multiple Gestations: rely on blood work AND Nuchal Translucency together: "In twin pregnancies, the most accurate screening for aneuploidy is achieved with cell-free DNA screening, although with weaker evidence and a higher rate of test failure due to lower fetal /placental fraction. First-trimester serum screening combined with nuchal translucency may also be considered in twin pregnancies.”

Normal Ultrasound Soft Markers

"The presence of specific ultrasound soft markers associated with fetal trisomy 21 (echogenic intracardiac focus) or trisomy 18 (choroid plexus cysts) at the time of the second-trimester ultrasound is not clinically relevant because of its poor predictive value, and such findings do not warrant further testing. The value of other soft markers, including mild ventriculomegaly, absent nasal bone, renal pyelectasis, thickened nuchal fold, and echogenic bowel, is weak in pregnancies at low risk for aneuploidy based on previous screening.” Found Isolated Ultrasound Soft Markers on 2nd Trimester Scan that are not clinically relevant (no further testing needed) include:

• EICF"echogenic intracardiac focus", especially with Trisomy21(down)
• CPCs"choroid plexus cysts", related again to trisomy 18 Other Soft Markers: mild ventriculomegaly, absent nasal bone, renal pyelectasis, thickened nuchal fold, echogenic bowel Value of these are weak in low-risk pregnancies.

First Prenatal Visit Timeline

• 1st prenatal visit to Primary Care in Nova Scotia and the 1st Trimester Dating Ultrasound should occur between 8-12 weeks, so as to confirm gestational age, and test babies with Serum because Mother Serum Test will not be applicable if multiples Tests for GDM if: Strong Risk factor, Preexisting DM, Aboriginal, PCOS, Renal Disease, Hemoglobinopathy, Use of Stroids, 1st Trimester Dating scan occurs between 8-12 weeks,
• and confirm gestational age, viability, number of fetuses, early anatomic assessment Multiple Gestations needs confirm chorionicity (Number of Sacks) and what is the nuchal translucency and
• Maternal Serum Test is not applicable for multiples

Early Maternal Serum Testing (MST)

• should occur 11-14 weeks and is to assess the Nuchal Translucency

• should be Offered to all patient regardless of age,

• a Second Trimester Maternal Serum Testing (MST)must occur between 14-22 weeks

• Integrated Maternal Serum Testing (IMST) should be done together, but one can do either by them self is prefered, if cost is a hinder

• Integrate to assess risk of early: fetal abnormalities and open fetal defects with maternal age

• should be integrated with a full Ultrasound with dating scan, if risk is on board

Estimated Risk for Chromosomal Abnormalities - General Population

• Trisomy - Is there an extra copy of one of The Fetal medicine/foundation tests are run on the chromosomes.
• Down Syndrome (Trisomy 21)
• Edward Syndrome (Trisomy 18)
• Patau Syndrome (Trisomy 13)

The Ultrasound is essential for Nuchal Translucency (NT)

The Blood tests must involve:

Maternal Serum Screening includes:

• hCG (human chorionic gonadotrophin)
• uE3 (unconjugated estriol
• AFP (alpha-fetoprotein)
• Inhibin A Genetic testing: needle used 4 amnio/CVS/fetal cord sampling

In the Ultrasound world: Abnormal Nuchal Translucency must reach more than more than 3.5mm to confirm/test is abnormal

NT Increased: 1- NT associated with cardiovascular, diaphragmatic abdo, renal- memorize

Ultrasound measurements

the inner border of the horizontal line of the callipers placed ON the liner that defines the niche translucency thickness(innerline). +turn down the gain, with turning down the gain and zooming(pre and post) always

Maxiumun number is more than one. So what you should do in detail The umbilical cord may be around the fetal neck! That can happen in about 4-5% cases After 14 weeks the fetal lymphatic system is developed suffcient to drain away excess fluid!

Testing Parameters

PAPP-A (pregnancy blood test)

• PAPP-A Is produce by the placenta!

PAPP-A is decreased if something is bad with pregnancy(Trisomy 21 and Edward Syndome) Quad screen: Maternal serum:Know: 15-22 weeks weeks to know(68% with Down, 69% with triplet)- is know it must be: AF, Estriol, HcG, Inhitin A What will these do: AFP- for open neural tube defects/decrease to know down/13& 18), estriol (dec Down), hgc (dec Down or Inc Tri21!

AFP stands for: Alpha feroprotein(glycoprotein from the Yolk sac, found within amnio fluid into ferel urine(trace to maternal blood) E3- formed: liver- dec. in down HCG-produced= trophoblast , elevated is 21 Inhibitn A: Helps improve the detection rate of Down Syndrome these parameters to KNOW values all do/change together or independently with the patient in question