Obstetrics & Gynecology

First-trimester crown-rump length ultrasound is the most accurate dating method when LMP is unknown or unreliable.

Cell-free DNA (noninvasive prenatal testing) has the highest sensitivity for trisomy 21 and can be done from 10 weeks.

0.4 mg (400 mcg) daily; 4 mg daily if prior neural tube defect pregnancy.

At 18-20 weeks gestation to assess fetal structures.

At 28 weeks and again within 72 hours postpartum if the infant is Rh-positive; also after any bleeding/trauma event.

Indirect Coombs test (indirect antiglobulin test) on maternal serum.