5 mm = Approx. 5 weeks (should be seen on sonogram)
GS increases in size by 1 mm/day
Delays in GS Size:
If the difference between Mean Sac Diameter (MSD) and Crown Rump Length (CRL) is less than 5 mm, correlates with higher risk (e.g., Down syndrome).
Cardiac Activity Evaluation
Cardiac activity should be visible by:
7 mm CRL and MSD of 25 mm
ALWAYS by 9 mm CRL
Older criteria indicated active monitoring at 5 mm CRL and MSD of 16 mm.
Yolk Sac (YS) Evaluation
Maximum size should be 5-6 mm by 10 weeks for normal
Increased risk of failure correlates with larger YS sizes; once cardiac activity is noted, the YS significance diminishes.
Amnion Evaluation
Double bleb appearance possible at 5-7 weeks.
The amnion should not be enlarged relative to CRL.
Presence of an amnion without an embryo indicates abnormality (empty amnion).
Fetal Pole Growth
The fetal pole should grow similarly to the amnion; discrepancies can indicate negative outcomes.
First Trimester Cardiac Activity
The embryonic heart begins beating at around 40 days post-menses or 4.5-5 menstrual weeks.
Visualization on ultrasound occurs around 5.5-6 menstrual weeks.
Bradycardia defined as:
<100 BPM before 6.2 weeks
<120 BPM between 6.5-7 weeks.
Placenta Development
Placenta may be seen as early as 6-7 weeks, with reliable observation at 10-12 weeks.
Fetal contribution primarily from chorion frondosum.
First Trimester Complications Overview
Spontaneous Abortions: Occurs in 15% of pregnancies.
Vaginal bleeding in 25% of patients, more common in the first few weeks.
Common clinical findings include:
Severe pain, contractions, dilated cervix, vaginal bleeding.
Maternal Age Risks
Women older than 35 years have 1.5-5 times higher risk of Down syndrome.
Incidence of Down syndrome increases from 1:1000 at age 30 to 1:30 at age 45+.
High-Risk Maternal Factors
Smoking, age, history of miscarriage, structural uterine anomalies, previous abnormal pregnancies.
Structural Uterine Abnormalities
Synechiae (scarring & adhesions) can lead to complications.
Fibroids and septate uteri are associated with increased abortion rates.
Recurrent Miscarriages
Defined as 3+ consecutive first trimester miscarriages.
Most occur before 8 weeks; ultrasound assessments are advised around 8 weeks.
Teratogens and Fetal Anomalies
Environmental factors, including teratogens, account for 10% of anomalies.
Early exposure (up to 5 weeks) can lead to demise or no effect; 5-10 weeks is critical for organogenesis, resulting in severe anomalies.
Differentiating Threatened Abortions
Important entities include:
Ectopic pregnancy
Trophoblastic disease
hCG levels crucial for differentiation; a quantitative blood test can assist.
Clinical Symptoms of Pending Miscarriage
Severe pain, cramping, dilated cervix, vaginal bleeding.
Abnormal Trophoblastic Appearance
Look for distorted sac; if GS ring
Blighted Ovum
Identifiable by a GS with no embryo and presence of YS (anembryonic sac).
Pseudogestational Sac Concerns
Differentials include normal early IUP, abnormal IUP (anembryonic), and PUL (positive hCG but not visible on ultrasound).
Role of Doppler Ultrasound
Useful for differentiating between pseudogestational sac and IUP.
Irregular Convex Bludge
Also known as chorionic bump; may mimic a non-viable embryo.
Subchorionic Hematoma
Intrauterine bleeding associated with IUP; may lead to pregnancy failure depending on size.
First Trimester Anomalies
Serious conditions such as anencephaly, encephalocele, holoprosencephaly, and abdominal wall defects (omphalocele vs. gastroschisis) can be visualized.
Specific Anomalies:
Anencephaly: Lack of skull development; indeterminate fetal head shape.
Encephalocele: Herniation of cranial content through cranial defects.
Holoprosencephaly: Failure of prosencephalon cleavage leading to cystic deformities.
Abdominal Wall Defects:
Omphalocele: Mass contained by peritoneal membrane, smoother appearance.
Gastroschisis: Irregular contour with bowel contents not covered.
Cystic Hygroma
Associated with conditions like Down syndrome and Turner syndrome; characterized by large cystic spaces in posterior fetal structures.
Amniotic Band Syndrome
Can cause various deformities and tissue death due to entanglement of fetal parts.
Pitfalls in Developmental Assessment
Rhombencephalon visible between 7-9 weeks should not be confused with Dandy Walker malformation.
Physiologic bowel herniation occurs from weeks 8 to 12.
First Trimester Labs
hCG Levels: Used to evaluate pregnancy status; threshold for visualization of IUP is typically between 1000-2000 mIU/ml, with newer standards at 3000-4000 mIU/ml.
Lab Screening for Aneuploidy
Triple Screen: Tests for AFP, hCG, and estriol levels.
Sensitivity of 81% without ultrasound dating.
Quad Screen: Add inhibin-A to evaluate additional markers.
NIPT: Non-invasive prenatal testing for fetal cells in maternal blood, excellent sensitivity.
Measurement Assessments
Crown Rump Length (CRL): Most accurate measurement for dating pregnancy, especially early.
Mean Sac Diameter (MSD) determined by averaging inner measurements.
Should be paired with clinical judgment and hCG levels for best accuracy.
Nuchal Translucency Measurements
Critical window for assessment between 11-14 weeks; increased NT thickness can indicate chromosomal abnormalities.
Cervical Evaluations
Monitor cervical length; cerclage may be necessary for shortened cervix (below 3 cm).