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).