Obstetrics
Domain overview
The Obstetrics domain tests prenatal care, antepartum surveillance, labor management, and obstetric emergencies. The exam emphasizes ACOG-supported timing of screening and evidence-based management of common complications.
Core rules
- Establish gestational age with first-trimester ultrasound (crown-rump length 7–13 6/7 weeks is most accurate). Naegele’s rule (LMP + 280 days) is used only if ultrasound dating agrees within 7 days in the first trimester.
- Screen for aneuploidy with cell-free DNA from 10 weeks or combined first-trimester screen. Screen for gestational diabetes at 24–28 weeks using a 1-hour 50-g glucose challenge; if abnormal, confirm with a 3-hour 100-g GTT.
- Culture for group B streptococcus at 36 0/7–37 6/7 weeks. Give intrapartum penicillin for positive, unknown with risk factors, or prior GBS-affected infant.
- Give RhoGAM to RhD-negative women at 28 weeks and within 72 hours postpartum if the infant is RhD-positive, plus after any sensitizing event.
- Interpret fetal heart tracings: early decelerations = head compression; variable = cord compression; late = uteroplacental insufficiency.
Worked example
A 34-week gravida has spontaneous rupture of membranes and regular contractions. The tracing shows repetitive late decelerations with minimal variability. Reasoning: late decelerations signal fetal hypoxemia from uteroplacental insufficiency. Start intrauterine resuscitation-left lateral position, IV crystalloid, and stop oxytocin if running. Do not give tocolytics. If decelerations persist, expedite delivery.
Traps
Confusing variable with late decelerations costs marks; variable decelerations are abrupt and often correct with position change. Also, do not use second-trimester ultrasound to redate a pregnancy when a first-trimester scan exists-first-trimester dating is more accurate. Failing to give GBS prophylaxis when indicated is another common error.