Pregnancy Complications and Fetal Well-Being: Placenta, Fluid, Twins, Growth
Placenta and cord
Placenta previa is placental tissue covering the internal os; low-lying placenta lies near but not over it, and most apparent low-lying placentas at the 18–22 week scan resolve by term as the lower segment forms. Placenta accreta spectrum is suggested by previa with prior cesarean, placental lacunae, loss of the clear zone, and bladder wall irregularity. Abruption usually appears as a retroplacental or subchorionic hematoma but is often a clinical diagnosis. Vasa previa shows vessels crossing the internal os over a velamentous cord insertion - confirm with color Doppler and avoid digital vaginal examination.
Amniotic fluid
Use either amniotic fluid index (oligohydramnios under 5 cm, polyhydramnios over 24 cm) or single deepest pocket (under 2 cm and over 8 cm). Do not mix thresholds between methods. Oligohydramnios suggests rupture, uteroplacental insufficiency, or renal agenesis; polyhydramnios suggests diabetes, anencephaly, esophageal atresia, or twin-twin transfusion.
Multiple gestation
Chorionicity is most accurately assigned in the first trimester. Dichorionic twins show a thick membrane with a lambda or twin-peak sign; monochorionic twins show a thin T-sign membrane. Monochorionic pregnancies require surveillance for twin-twin transfusion (oligo-poly sequence), selective growth restriction, twin anemia-polycythemia sequence, and twin reversed arterial perfusion.
Growth and Doppler
Estimated fetal weight under the 10th centile is small for gestational age; under the 3rd centile with abnormal Doppler indicates growth restriction. Umbilical artery absent or reversed end-diastolic flow, middle cerebral artery brain sparing, and ductus venosus a-wave reversal indicate worsening status.
Rule of thumb
An MCA peak systolic velocity over 1.5 MoM suggests fetal anemia - the opposite direction of change from growth restriction, where MCA indices fall.