Predisposing factors:- Advanced maternal age (> 35 years)- Previous multiple pregnancy- Use of fertility enhancing treatments- Maternal family history of dizygotic twins

Dizygotic twins:- ⅔ of all twin pregnancies- Fertilization of two oocytes with two spermatozoa- Dichorionic-diamniotic

Monozygotic twins:- Dichorionic-diamniotic (∼ 20-30%; separate day 0-3): Each twin has an individual amniotic sac and placenta.- Monochorionic-diamniotic (∼ 70%; separate day 4-8): The twins share a placenta and have individual amniotic sacs.→ Complications: Twin-twin transfusion- Monochorionic-monoamniotic (∼ 1-5%; separate day 9-12): The twins share both the placenta and amniotic sac.→ Complications: Cord entanglement, conjoined twins

Diagnostics:- Fundal height and abdominal girth are unusually large for the gestational age.- Two or more fetal heart rates can be heard on auscultation.- Ultrasound→ Dichorionic twins: lambda sign→ Monochorionic twins: T-sign

Prenatal care:- Frequent early evaluations for monochorionic twins to assess for twin-twin transfusion syndrome- From the 32nd week of pregnancy: weekly prenatal care visits, including ultrasound, to monitor fetal growth