Pregnancy Trimesters

Overview: Developmental Timetable

First Trimester (Weeks 1-13)

Fetal Development

• Week 1-3: Tubal migration, blastocyst formation, implantation, bilaminar disc, yolk sac, and start of neurulation.
• Week 4: Folding of the embryo, neurulation concluded, basic body shape established.
• Weeks 5-8: Organogenesis - formation of all essential external and internal organs; limb buds elongate. By week 8 (Stage 23), eyelids fuse and external genitalia begin differentiation. Crown-rump length: ~27-31 mm.
• Weeks 9-13: Transition to fetal period - organ growth begins. By 9-12 weeks, CRL is 5-8 cm and weight is 10-45 g.

Maternal Changes

• hCG rises rapidly - detectable from 7 days post-fertilization; levels normally rise ~40% per day in early pregnancy. - Berek & Novak's Gynecology
• Nausea and vomiting ("morning sickness") are common; symptoms can occur any time of day. About 50% of pregnant women experience morning nausea. - Creasy & Resnik's Maternal-Fetal Medicine
• Cardiovascular: Peripheral vascular resistance begins to fall significantly in the first trimester. Cardiac output starts rising, primarily through increased stroke volume. - Morgan and Mikhail's Clinical Anesthesiology
• The uterus remains within the pelvis until the end of the first trimester (rises above the symphysis pubis by ~12 weeks).

Second Trimester (Weeks 14-26)

Fetal Development

• Organ growth and functional maturation continue.
• Surfactant production begins around week 20, though amounts are minimal before 24 weeks - this is critical for lung maturity and neonatal survival. - Mulholland and Greenfield's Surgery
• External genitalia differentiation is sex-specific in this period.

Maternal Changes

• The uterine fundus reaches the level of the umbilicus by 20 weeks.
• Peripheral vascular resistance reaches its nadir in the mid-second trimester. - Morgan and Mikhail's Clinical Anesthesiology
• Cardiac output peaks (primarily from stroke volume increase peaking at 28-31 weeks). Renal blood flow increases by ~40%; uterine blood flow rises from 1% to 15% of cardiac output. - Medical Physiology
• Blood pressure typically is at its lowest point in pregnancy during this trimester.
• The anatomic survey ultrasound is performed in the second trimester - it does not replace first-trimester scanning, as some anomalies only become apparent later. - Creasy & Resnik's Maternal-Fetal Medicine
• Nausea usually resolves by mid-second trimester for most women.
• Surgical procedures (e.g., cholecystectomy) are safest in this trimester: risk of spontaneous abortion is lower than first trimester, and preterm labor risk is lower than in the third. - Mulholland and Greenfield's Surgery

Third Trimester (Weeks 27-40)

Fetal Development

• Rapid weight gain and fat deposition.
• By the beginning of the third trimester, fetal viability is possible if born prematurely, though lung maturity is the key limiting factor. - Mulholland and Greenfield's Surgery
• Surfactant levels increase sufficiently after 34-36 weeks. Antenatal corticosteroids (betamethasone or dexamethasone) can accelerate lung maturity if preterm birth is anticipated before 34 weeks.

Maternal Changes

• The uterus reaches the upper abdomen by 36 weeks, nearly contacting the liver.

Enlarging uterus at 12, 20, and 36 weeks. - Mulholland and Greenfield's Surgery

• Supine hypotension: The gravid uterus can compress the inferior vena cava when lying flat, reducing cardiac preload. Left lateral decubitus positioning is recommended.
• In the third trimester, cardiac output rises again primarily through heart rate (rather than stroke volume, which has already peaked). - Fuster and Hurst's The Heart, 15e
• Blood volume at term is expanded by ~1000-1500 mL above baseline (physiologic dilutional anemia is normal). - Miller's Anesthesia
• Third trimester surgery carries significant risk of inducing preterm labor. - Current Surgical Therapy 14e
• Braxton Hicks contractions (irregular, painless) become more noticeable.
• Engagement of the fetal head into the pelvis typically occurs in the last few weeks in primigravidas.

Key Physiological Changes Across All Trimesters (Summary)

Viability and Prematurity Milestones