Multiple Pregnancy (Twins, Triplets or More)

Types of twins

Fraternal and identical

• Fraternal (dizygotic) twins come from two eggs fertilized by two sperm. They are as alike as any siblings and can be different sexes. About two-thirds of twins are fraternal.
• Identical (monozygotic) twins come from one fertilized egg that splits. They share the same genes.

Placentas and sacs: chorionicity

Doctors care most about whether the babies share a placenta (chorion) and an inner sac (amnion), as this drives the risks.

Chorionicity is most accurately determined by ultrasound before 14 weeks, by the shape where the membranes meet the placenta.

Causes and risk factors

• Fertility treatment: IVF with more than one embryo, and ovulation-stimulating medicines
• Older maternal age: women over 35 release more than one egg more often
• Family history of fraternal twins on the mother's side
• Previous pregnancies, taller height and higher body weight
• Ethnicity: fraternal twins are more common in Black women and less common in Asian women

Identical twinning happens at a similar rate worldwide and is not inherited.

Twin pregnancy symptoms and how it is found

Many women have no particular signs. Some notice stronger morning sickness, more tiredness, a uterus that measures large for dates, or earlier movement. hCG levels tend to be higher. Almost all multiple pregnancies are diagnosed on the first ultrasound.

Risks in a multiple pregnancy

For the mother

• Preeclampsia and high blood pressure, several times more likely than with one baby
• Gestational diabetes
• Severe nausea and vomiting (hyperemesis gravidarum)
• Anemia from higher iron needs
• Blood clots, back pain and breathlessness from a larger uterus
• Cesarean delivery and heavy bleeding after birth (postpartum hemorrhage)

For the babies

• Preterm birth: more than half of twins are born before 37 weeks, and the risk rises with each extra baby
• Growth restriction, including one twin growing much less than the other (selective growth restriction)
• Twin-to-twin transfusion syndrome (TTTS): in about 10 to 15% of MCDA twins, blood vessels in the shared placenta send blood unevenly, so one twin gets too much and the other too little. It is treated with fetoscopic laser surgery in specialist centers.
• Twin anemia-polycythemia sequence (TAPS): a slower imbalance of blood between twins
• Cord entanglement in MCMA twins
• Birth defects and stillbirth occur more often, particularly in identical twins

Prenatal care

• Early ultrasound to date the pregnancy and determine chorionicity
• More frequent scans: shared-placenta twins usually have ultrasounds every 2 weeks from about 16 weeks to look for TTTS; separate-placenta twins about every 4 weeks from 20 weeks to check growth
• Low-dose aspirin from 12 weeks is recommended to lower preeclampsia risk, as advised by your obstetric team
• Iron and folic acid supplements and anemia checks
• Nutrition: recommended weight gain for twins with a normal starting BMI is about 37 to 54 lb (17 to 25 kg)
• Screening tests: first-trimester combined screening and cell-free DNA tests work in twins, with some limits
• Care with a maternal-fetal medicine specialist, especially for monochorionic twins and higher-order multiples

Bed rest and routine cervical stitches have not been shown to prevent preterm birth in twins, and bed rest raises the risk of blood clots.

When are twins delivered?

ACOG recommends timing delivery by type to balance prematurity against late stillbirth risk:

Vaginal birth is a reasonable choice for twins when the first twin is head-down and there are no other complications. The second twin can sometimes be delivered by breech extraction or may occasionally need a cesarean after the first is born vaginally.

After the birth

Twins are more likely to need neonatal intensive care. Breastfeeding twins is possible, and lactation support helps. Parents of multiples face more sleep loss and stress, and have a higher risk of postpartum depression, so practical help matters.

Frequently asked questions

What are the risks of a twin pregnancy?

The main risks are preterm birth, preeclampsia, gestational diabetes, anemia, poor growth of one or both babies and cesarean delivery. Identical twins sharing a placenta can develop twin-to-twin transfusion syndrome. Close monitoring with extra ultrasounds catches most problems early enough to act.

How can you tell if twins are identical?

Twins of different sexes are always fraternal. Twins sharing a placenta on early ultrasound are identical. Same-sex twins with two placentas can be either type, and only a DNA test after birth confirms it. For pregnancy care, placenta sharing matters more than whether twins are identical.

When are twins usually born?

Average twin birth is around 35 to 36 weeks. Planned delivery for uncomplicated twins with separate placentas is usually at 38 weeks, and for twins sharing a placenta between 34 and 37 weeks. Many twins arrive earlier due to preterm labor or complications.

What is twin-to-twin transfusion syndrome?

It is a complication of identical twins sharing a placenta, where blood flows unequally between them through connecting vessels. One twin produces too much fluid and the other too little. It is found on regular ultrasounds from 16 weeks and treated with laser surgery or other procedures in specialist centers.

How much weight should you gain with twins?

US guidance for twins suggests about 37 to 54 lb (17 to 25 kg) for women starting at a healthy weight, 31 to 50 lb (14 to 23 kg) if overweight, and 25 to 42 lb (11 to 19 kg) if obese. Your care team will tailor this and check your iron levels.