Obstetric Topic 31. Multiple gestation

I. Definition and Classification

Definition

• Multiple gestation: pregnancy with 2 or more embryos/fetuses in the uterus simultaneously.
• Key practical classification: zygosity, chorionicity and amnionicity.
• Chorionicity is more important than zygosity for risk assessment.

Types of Twins

1. Dizygotic / fraternal twins:
   • Incidence: about 2/3 of twin pregnancies.
   • Origin: 2 oocytes fertilized by 2 spermatozoa in the same cycle.
   • Genetics: different, like ordinary siblings; may be different sex.
   • Placenta/sacs: dichorionic-diamniotic, but placentas may fuse.
2. Monozygotic / identical twins:
   • Incidence: about 1/3 of twin pregnancies; baseline rate is relatively constant.
   • Origin: division of one fertilized oocyte.
   • Genetics: identical / same sex in most cases.
   • Chorionicity/amnionicity depends on timing of division.

Timing of Monozygotic Division

1. 0-3 days: dichorionic-diamniotic twins.
2. 4-8 days: monochorionic-diamniotic twins; most monozygotic twins.
3. 8-12 days: monochorionic-monoamniotic twins.
4. >13 days: conjoined twins.

Risk Factors

• Dizygotic twinning risk increases with advanced maternal age, high parity, family history, previous multiple pregnancy and assisted reproductive techniques.
• Monozygotic twinning is less influenced by maternal factors; ART slightly increases risk.

II. Diagnosis and Surveillance

Diagnosis

• Clinical suspicion: uterus/fundal height larger than dates, excessive weight gain, severe early nausea/vomiting, many fetal parts or more than one fetal heart rate.
• Ultrasound: confirms number of fetuses, viability, chorionicity and amnionicity.
• First-trimester ultrasound: determine chorionicity and amnionicity as early as possible.
• Lambda / twin-peak sign: dichorionic pregnancy.
• T-sign: monochorionic-diamniotic pregnancy.
• Different fetal sex: dizygotic and therefore dichorionic.

Antenatal Care

• Multiple gestation = high-risk pregnancy → more frequent prenatal visits and ultrasound follow-up.
• Assess fetal growth, amniotic fluid, presentation, maternal BP/urine, anemia and diabetes risk.
• Iron and folate supplementation; nutrition/weight-gain counseling.
• Dichorionic twins: serial growth scans.
• Monochorionic twins: closer ultrasound surveillance for TTTS from about 16 weeks.
• Monoamniotic twins: specialist care because of cord entanglement/compression risk.

III. Complications

Maternal Complications

• Anemia, hyperemesis, excessive weight gain.
• Hypertensive disorders, especially preeclampsia.
• Gestational diabetes.
• Polyhydramnios, preterm labor, PPROM.
• Cesarean delivery, postpartum uterine atony and postpartum hemorrhage.

Fetal / Neonatal Complications

• Prematurity and low birth weight.
• FGR/IUGR, growth discordance, congenital anomalies.
• Malpresentation and cord prolapse.
• Monoamniotic twins: cord entanglement/compression.
• Monochorionic-specific risks: TTTS, selective FGR and co-twin injury after single fetal demise.

Twin-to-Twin Transfusion Syndrome

• TTTS: unbalanced vascular anastomoses in a monochorionic placenta → net blood flow donor → recipient.
• Donor twin: hypovolemia, anemia, oligohydramnios, small bladder, IUGR.
• Recipient twin: hypervolemia, polycythemia, polyhydramnios, cardiomegaly/heart failure, hydrops.
• Classic ultrasound: oligohydramnios in donor + polyhydramnios in recipient.
• Treatment: fetoscopic laser ablation of anastomosing vessels in a fetal therapy center.

IV. Delivery

Timing

• Uncomplicated dichorionic-diamniotic twins: planned delivery around 37-38 weeks depending local protocol.
• Uncomplicated monochorionic-diamniotic twins: planned delivery earlier, commonly 36-37 weeks depending protocol and complications.
• Monoamniotic twins: planned cesarean around 32-34 weeks after corticosteroids because of cord accident risk.
• Deliver earlier for preeclampsia, fetal compromise, severe FGR, TTTS, PPROM, abruption or maternal instability.

Mode of Delivery

• Decide by presentation of first twin, gestational age, fetal weights/discordance, fetal status, amnionicity and operator skill.
• Vaginal delivery is reasonable for diamniotic twins when first twin is cephalic and no contraindication exists, with immediate cesarean capability.
• After first twin is born: reassess second twin lie/presentation/FHR; aim to deliver within about 30 min when possible.
• Second twin malpresentation: version, breech extraction or cesarean depending fetal condition and operator expertise.
• Cesarean section: monoamniotic twins, first twin non-cephalic in many protocols, conjoined twins, fetal compromise or standard obstetric indications.
• Prepare for postpartum hemorrhage: uterine overdistension → atony.