JN

First Trimester Overview and Key Concepts

First Trimester: Key Concepts

Conceptual Stages of Pregnancy

  • Conceptus Stage: Up to 5 weeks of pregnancy.
  • Embryonic Stage: 6-10 weeks of pregnancy.
  • Fetal Stage: 9 weeks and onward (38+ weeks).
  • Trimester Breakdown:
    • 1st Trimester: 1-12 weeks
    • 2nd Trimester: 13-28 weeks
    • 3rd Trimester: 29-38 weeks

Normal Gestational Sac (GS) Criteria

  • GS Size:
    • 2-3 mm = 4 weeks of menstrual age
    • 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).