Antepartum Care and Prenatal Diagnosis
Core concepts
Routine prenatal care is a schedule of visits, screenings, and education. Know the standard prenatal screening timeline: first-trimester combined screening (nuchal translucency, PAPP-A, free beta-hCG) at 11–14 weeks; cell-free DNA from 10 weeks; second-trimester quad or maternal serum screen at 15–22 weeks; anatomy survey at 18–22 weeks; gestational diabetes screening at 24–28 weeks; group B streptococcus at 36–37 weeks. Understand the difference between screening (risk estimate) and diagnostic testing (chorionic villus sampling, amniocentesis, with their miscarriage risks and timing). Fetal aneuploidy risk rises with maternal age, and the choice of test depends on desired information, timing, and invasiveness.
Traps
Examiners love to test whether a screening test result changes management without confirmation. A positive screen is not a diagnosis. Another trap is timing: nuchal translucency is invalid outside 11–14 weeks, and cell-free DNA is a screening test, not diagnostic, despite high sensitivity. Also distinguish dating ultrasound (crown-rump length in the first trimester is most accurate) from growth assessment later.
Rule of thumb
When a question asks "next step" after an abnormal screen, the answer is usually diagnostic testing with counseling, not immediate treatment or repeat screening.