Twins account for about 3.5% of births in the United States. When you hear the phrase, “twins run in my family, so I’m likely to have twins” it is only true if fraternal twins on the mother’s side have occurred. Twin pregnancies are actually twice as common in women over the age of 35 than women 25 and under. The rate of multiple births has increased in recent years due to the increased use of fertility medication and treatment. In fact, the twin birth rate in the United States rose 76% from 1980 through 2009!

Twins can occur in two different ways. Fraternal or dizygotic twins are the result of two separate eggs that are fertilized by two different sperm. Each embryo will have it’s own placenta, umbilical cord and amniotic sac. This type of twinning occurs in nearly 70% of all twin pregnancies.

When a single embryo splits, identical or monozygotic twinning occurs (30% of all twin pregnancies). There are a variety of different ways identical twins can look on an ultrasound based on when the embryo splits. The earlier the split occurs (up to 72 hours post fertilization), the more separate the gestational sacs and placentas will be. If the embryo split occurs as late as 6 – 9 days after fertilization, it is likely the twins will share a single amniotic sac with no dividing membrane between them, called monoamniotic twinning. Only 1% of twin pregnancies are monoamniotic. However, monoamniotic twin gestations are a cause for concern due to the increased risk of umbilical cord entanglement. In addition, identical twins are at risk for twin-to-twin transfusion syndrome (TTTS), uneven weight gain in utero, congenital anomalies and premature delivery.

It is important to distinguish the type of twin pregnancy as early as possible. This is done with an ultrasound examination as early as 6 weeks gestation.

Risks to the mother during pregnancy with multiples include anemia, preeclampsia, hypertension, gestational diabetes, and preterm labor. Due to the increased risk factors associated with having twins, you will see your practitioner more frequently during your pregnancy. In addition, you will generally have more frequent ultrasounds to assess fetal development. In the second trimester, your practitioner will want to assess the length of your cervix through ultrasounds because the risk of an incompetent cervix (definition: add to glossary: Dilation of the cervix without labor often resulting in miscarriage or premature birth) is more common when carrying multiples. During the third trimester, your practitioner will monitor you closely for signs of preterm labor. 36 weeks gestation is considered full term for twins and you may be advised to go on bed-rest to help prolong the pregnancy. At 36 weeks gestation, weekly or bi-weekly nonstress tests (NST) and/or biophysical profiles are usually performed to monitor fetal well being.

Twins are typically born via cesarean section. The difficult presentations of the babies at birth – such as breech and transverse lie – can result in fetal entrapment, fetal asphyxia and birth trauma. In full-term singleton pregnancies the majority of babies rotate into the head down position. Due to the space restriction caused by two babies, it is relatively uncommon for a twin pregnancy to have both babies in the head down position, making vaginal birth a potentially safe option.