Antepartum Care
Core concepts
Accurate dating anchors every antepartum decision. Naegele's rule (first day of the last menstrual period + 7 days − 3 months) assumes a 28-day cycle; a first-trimester crown–rump length or an early second-trimester ultrasound supersedes an uncertain LMP. Know the routine visit cadence (every 4 weeks to 28, every 2 weeks to 36, weekly thereafter) and the standard screening timeline: CBC, blood type/Rh and antibody screen, rubella and varicella immunity, HBsAg, HIV, syphilis, gonorrhea/chlamydia, and urine culture at intake; aneuploidy screening in the first or second trimester; anatomy survey at 18–22 weeks; gestational diabetes screening at 24–28 weeks; Tdap at 27–36 weeks; group B streptococcus culture at 36 0/7–37 6/7 weeks; Rh immunoglobulin at 28 weeks for unsensitized Rh-negative clients.
Also master nutrition and weight-gain ranges by prepregnancy BMI, folic acid supplementation, danger-sign teaching, and the recognition and referral thresholds for hypertensive disorders, preterm labor, and fetal growth restriction. Low-dose aspirin is recommended for people at elevated preeclampsia risk.
Traps
Questions blur screening with diagnosis (a positive screen needs a diagnostic test), misplace the GBS window, or imply ultrasound and LMP are interchangeable. Watch for "routine" labs offered at the wrong gestational age and for Rh prophylaxis given to an already sensitized client.
Worked example
LMP March 5: add 7 days (March 12), subtract 3 months → EDD December 12. If a 9-week ultrasound differs by more than about a week, redate by ultrasound.
Rule of thumb
When dating is uncertain, the earliest accurate ultrasound wins; when a screening result is abnormal, the next step is a diagnostic test and counseling-not immediate treatment.