First Trimester Ultrasound: Dating Pregnancy

First Trimester Normal Exam: Dating the Pregnancy

• Ultrasound is most accurate for gestational age (GA) determination in the first and early second trimester, specifically before 23 weeks, within spontaneous conceptions.
• Crown-rump length (CRL) is the best parameter for determining gestational age.
• In cases with multiple first-trimester scans, the earliest measurement of Mean Sac Diameter (MSD) or CRL should be used to determine gestational age.
• When using CRL to determine gestational age, the earliest CRL should be equivalent to at least 7 weeks (10 mm).
• 3 measurements are needed to assess CRL accurately.
• CRL is most precise if it is less than or equal to 84.0mm.
• Use the mid-sagittal plane of the fetus to measure CRL, ensuring the visibility of the fetal nasal bone and spine.
• Use Head Circumference (HC) to determine gestational age if CRL is greater than 84 mm
• Head Circumference is better to look at after CRL of 84 than Biparietal Diameter because HC is more reliable at that stage.

First Trimester Anatomy: Fetal Head

• Cranial bones are ossified during this stage
• There is no skull defect in normal anatomy.
• Lateral ventricles are filled with echogenic choroid plexus

Fetal Head SA:

• CP = choroid plexus
• Lat. V = lateral ventricle
• 3rd V = 3rd ventricle
• Thal. = thalamus
• Cer. Ped. = cerebral peduncles
• AS = aqueduct of Sylvius
• Falx is the line dividing the two sides of the brain and it is symmetrical

Fetal Head: Posterior Fossa in Sagittal Plane

• The following structures should be visible:
• T = thalamus
• M = midbrain
• B = brainstem
• MO = medulla oblongata
• NT = nuchal translucency
• Note the nasal bone, palate, and mandible.

Fetal Face

• Maxilla
• Mandible

Normal Fetal Situs

• The stomach is seen on the left side of the fetus.
• the 4-chamber heart view points to the left side of the fetus.

Fetal Spine

• When viewed in a longitudinal plane, the fetal spine will look like two parallel railroad tracks reflecting the ossification centers of the vertebral bodies.
• The individual vertebrae are tiny.

Fetal Abdomen

• Assess the umbilical cord insertion into the fetal abdomen
• Fetal stomach on the left

Midgut Herniation

• Midgut herniation is considered normal physiology at this stage.

Genitourinary Tract

• Genitalia assessment is not reliable in the first trimester
• Fetal kidneys normally appear echogenic
• The small fetal bladder can usually be identified as a ‘snichoic’ (anechoic) fluid-filled structure
• Use color Doppler with caution in this area

Upper and Lower Extremities (Limbs)

• There should be two arms and two legs.
• Ensure proper orientation of the limbs.
• Verify that the bones are ossified.
• Assess for the presence of each bony segment in each limb, including the femur, tibia-fibula, humerus, ulna, and radius

Placenta and Cord Insertion

• By the late first trimester, the placenta is visible
• The umbilical cord insertion site should be seen before the end of the first trimester

First Trimester Sonography: Patient Preparation

• A distended urinary bladder is needed for transabdominal (TA) ultrasound
• An empty urinary bladder is needed for transvaginal (TV) ultrasound
• With transabdominal ultrasounds, a distended bladder displaces bowel and provides a good acoustic window

Options for achieving a full bladder include:

• Drinking one liter of clear fluid without voiding
• Foley catheter insertion
• Intravenous drip
• An overdistended bladder may cause extreme patient discomfort.
• Compression from an overdistended bladder can distort anatomy.
• Structures may be pushed out of the field of view or May obscure visualization of an early gestational sac
• May artificially elongate the cervix, mimicking placenta previa

Obstetrical Patient History: Key Information

• Why the exam is done, including routine screening, dating, maternal problems, etc.
• Patient symptoms such as pain, bleeding, poor growth, or lack of fetal movements
• Age of the pregnant patient
• Gravidity (number of pregnancies)
• Parity (number of births)
• Last menstrual period (LMP)
• Previous pathology, family history of anomalies, previous surgeries
• Maternal serum testing, amniocentesis, chorionic villous sampling

Goals of First Trimester Sonography

• Visualize and locate the gestational sac either in the uterus, outside the uterus, or both
• In cases of multiple gestations, verify if one twin is inside the uterus and one outside
• Identify embryonic demise and anembryonic gestations early on.
• Determine if an embryo is present along with a heartbeat
• Determine the number of embryos
• Determine chorionicity and amnionicity, if there is more than one embryo
• Estimate menstrual age of the pregnancy and do early diagnosis of fetal abnormalities

Weeks 1-2 of Pregnancy

• This corresponds to days 1-14 of the mother's menstrual cycle
• Menstrual phase is days 1-5, characterized by primordial ovarian follicles and a thin endometrium
• Follicular/Proliferative phase is days 6-13. During this phase, the dominant Graffian follicle reaches 2 cm, and the endometrium begins thin and grows to 4-8 mm
• Ovulation occurs on day 14 of the cycle, with the ovum release and a trilaminar endometrium

Weeks 3-4 of Pregnancy

• Fertilization takes place in the ampulla of the fallopian tube
• The conceptus (zygote) enters the uterus and becomes a morula, then a blastocyst
• The blastocyst implants into the endometrium.
• The primary yolk sac forms, gets pinched off, and forms the secondary yolk sac
• The placenta begins to develop in this period

Normal Intrauterine Gestation: Intradecidual Sign

• The earliest demonstration of a gestational sac by ultrasound is at 4 ½ – 5 weeks using transvaginal ultrasound
• The gestational sac will appear as a small gestational sac within the decidua
• There will be no fetal pole or yolk sac seen
• Gestational sac looks empty because it is too early to see embryo or yolk sac

Double Decidual Sign

• This demonstrates the 3 layers of the decidua in early pregnancy under ultrasound: decidua basalis, decidua capsularis, and decidua parietalis
• This can differentiate ectopic pregnancy from a normal IUP

Gestational Sac Appearance

• TA can see Gestational sac around 5 weeks (Mean Sac Diameter ≈ 5mm)
• TV can see Gestational sac around 4 1/2 weeks (Mean Sac Diameter ≈ 2-3mm)
• Gs grows approximately 1mm/day
• Round initially, becomes more oval as it enlarges
• GS implants immediately beneath the echogenic endometrial stripe
• May get distorted due to pressure from the bladder
• GS initially filled with chorionic fluid

Yolk Sac

• It is normally the first structure to be seen within the gestational sac at around 5 ½ weeks
• The presence within an early gestational sac can indicate IUP
• Upper limit of normal internal diameter is 5.6 mm
• It is connected to the embryo via vitelline duct, which is part of umbilical cord

Double Bleb Sign

• This is the earliest way under ultrasound to see amnion (at abour 5 1/2 weeks)
• There will be to blebs (bubbles): Amniotic cavity and yolk sac
• There will be an embryonic disc (more than 2mm) between the two blebs
• Amnion grows to meet chorion by 12 weeks, fuses at 16 weeks

Early Embryo and Cardiac Activity

• This will routinely be seen on US with the heart having activity by 5 weeks with CRL 5mm
• Can document with M-mode tracing (or video clip)
• If embryo is smaller than 5 mm, then may need to bring the patient back to record the heartbeat

Umbilical Cord

• Amnion expands and envelops the connecting stalk, yolk stalk, and allantois to form the umbilical cord by the end of the 6th week
• Do not apply Doppler to embryo unless necessary (ALARA)
• The umbilical cord contains 2 umbilical arteries (become the medial umbilical ligaments), 1 umbilical vein (becomes the ligamentum teres), Allantois (becomes the urachus and median umbilical ligament), and Yolk stalk – connects the primitive gut to the yolk sac

Physiologic Anterior Abdominal Wall Herniation

• Midgut Herniation is seen between 8-12 weeks
• Results in Normal herniation where the bowel is into the base of the umbilical cord
• See under sonographic appearance as a small echogenic mass protruding into the cord

Intracranial Cystic Structures

• This takes place around week 6
• Involves 3 primary brain vesicles form: prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain)
• You can see at between 6-8 weeks the normal intracranial cystic rhombencephalon under ultrasound on the posterior aspect of the embryonic head

Gestational Sac Size

• Occurs during approximately weeks 5-10
• The first structure seen is the GS prior to the yolk sac or embryo to first check
• Need to follow pregnancy until an embryo with heart activity is seen
• Can be very accurate to within one week of menstrual age
• Measurements need to be done from inner boarder to inner boarder
• Use can calculate using Mean sac diameter
  • MSD = L + W + H / 3, the gest sac then groth approximately 1mm / day

Crown Rump Length (CRL)

• This usually applies weeks 6-13, but can be seen as early as week 5 using transvaginal US
• Is is accurate to within 5-7 days of menstrual age
• Need measure from head to butt
  • Longest of 3 measurements is perfered or can average the 3 for more accurancy
  • Some US machines calculate average and need 3 CRL to get more accurate measurement
• Measurements gets less accurate towards the end of the 1st trimester due to fetal flexion and extension
• Must be taken under a mid-saggital plane and also ensure that measurement is not obliqued

##Multiple Gestation and US

Increased due to:

- ART and advanced maternal age
- high-risk patients
- ethnic origin
- heredity 
- maternal age and parity

Dizygotic twins:

- Are fraternal and result of two distinct fertilization events
- Have always two chorions and two amnions

Monozygotic twins:

- Are duplicates and arise from the same egg dividing in two
- Can have differect amnio / chorion levels dependant on when splits

Dichorionic and diamniotic (DCDA) twins: - Are the most reliable sign of dizgosity as will be under two sepereate sax and potentially also under differect sex as can be in fraturnal twins

Sonograhpic Amnio / Corion

• Must count all sax up to first ten weeks and then look for a planenta
• If has 2 sets the DC, if MC requires further testing

Time of seperation is key to determining final presentation:

• Morula Stage (1-4 days) = dichorionic-diamniotic (DCDA)
  • 76% of all twin pregnancies, 71-13% of MO
• Blastocyst Stage (4-8 days) = monochorionic-diamniotic (MCDA)
  • 30% of all twin pregnancies, 70-75% of MO
  • Implanted Blastocyst (1-2 weeks) = monochorionic-monoamniotic (MCMA)
• 1.5%

Diamniotic and Dichorionic signs under US:

• Inter-twin with two separate amnio and two chorion layers
• Can determine fetal sex as may be differect

Twin Loss can relate to factors such is:

• Prematurity - Delivery to early
• Intrauterine Growth Restriction (IUGR) - Twins may not grow at rate
• Monochorionic complications i