A twin pregnancy places additional demands and risks on a mother and her babies compared to a singleton pregnancy. Because of the increased risk for complications and poor outcomes, women with twins are more likely to deliver preterm than carry to term. Early diagnosis of the type of twins and close prenatal monitoring will guide prenatal care and delivery planning, even if a full term twin pregnancy is not expected.
The Chance of Delivering Twins at Term
Women pregnant with twins, according to Beischer and MacKay's Obstetrics, Gynaecology and the Newborn (page 114), have:
- About a 40 percent chance of carrying their pregnancy to early term - 37 weeks - or after
- A 60 percent chance of delivering preterm - at less than 37 weeks
- An 11 percent chance to deliver before 32 weeks of pregnancy
The reference also states the average duration of pregnancy for twins is 35 weeks (preterm), compared to 39 weeks (full term) for a single baby. Note that the American College of Obstetricians and Gynecologists (ACOG) defines term pregnancy as follows:
- Early term: 37 weeks to 38 weeks and six days gestation
- Full term: 39 weeks to 40 weeks and six days
A second ACOG reference (page 364) states that preterm birth is the most common complication of twin or other multiple pregnancies.
Recommendations for Timing of Delivery
Although preterm delivery of twins causes increased complications of prematurity, delivery after 37 to 38 weeks also increases the risk of fetal and newborn death, according to a 2014 article in Best Practice and Research. This compares to an increased risk after 42 weeks (post term) for singleton pregnancies.
When to Deliver Twins
The recommendation for when to deliver depends in part on whether the twins are in a single or separate sacs - diagnosable by early ultrasound. To decrease the risk of stillbirth, newborn death, and other complications of one or both twins, Brigham and Women's Hospital, a leading Harvard teaching hospital, recommends the following timing of delivery:
- Delivery at 37 weeks gestation: Uncomplicated identical (monozygotic - one egg) twins in a single chorionic sac and two separate amniotic sacs (monochorionic-diamniotic) and who share a placenta
- Delivery by 38 weeks: Uncomplicated identical twins and fraternal (dizygotic - two eggs) twins who are in two separate chorionic and two amniotic sacs (dichorionic-diamniotic) and don't share a placenta
- Delivery between 32 and 34 weeks: Identical twins who share a single chorionic, a single amniotic sac, and a placenta (monochorionic-monoamniotic); the mother should be hospitalized for monitoring between 24 to 28 weeks.
If there are any complicating factors, doctors might recommend delivery before 32 weeks.
Other Factors Influencing Delivery Timing
A variety of maternal and fetal factors can affect the chance of a twin pregnancy making it to term.
The Cleveland Clinic lists the following maternal factors that increase the chance of preterm delivery of twins and other multiple pregnancies:
- Gestational diabetes - more common in twin pregnancies
- Pregnancy-induced high blood pressure (PIH), which can increase as the pregnancy advances
- Preeclampsia - a syndrome of high blood pressure, water retention, and protein in the urine
- Placental abruption - separation of the placenta from the uterus before delivery
A previous history of preterm delivery is also a factor in preterm delivery of a twin pregnancy.
The following fetal complications of twin pregnancies can be diagnosed by pregnancy ultrasound. They increase the chance of delivering or needing to deliver before reaching term.
- Problems of monochorionic-monoamniotic twins: As described by Brigham and Women's Hospital, these identical twins, who share a placenta and a single chorionic and single amniotic sac, are at much greater risk for prenatal and delivery complications and death. This is because of increased chance of:
- Birth defects in one or both fetuses - such as neural tube defects
- Conjoined twins - when the babies in a single sac are joined at a part of their bodies such as the chest
- Entangled umbilical cords - the cords of twins in a single chorionic and a single amniotic sac can twist around each other, decreasing the supply of nutrients and oxygen and increasing the risk of death of both twins before, during, or after delivery.
- Twin-twin transfusion - in twins who share a single chorionic sac and a single placenta, a connection between their blood vessels in the placenta causes one twin to get a blood transfusion from the other. One twin then has too much blood and the other too little and both suffer complications from this.
- Intrauterine growth restriction (IUGR): There is an increased chance of preterm delivery when there is slow growth of one or both twins compared to normal growth of both, according to a study of twin pregnancies published in 2005 in the American Journal of Obstetrics and Gynecology.
- Intrauterine death of one twin - when one twin dies early in pregnancy, preterm delivery of the surviving twin might be necessary.
- Malpresentation - there is an increased chance of breech or other malposition of the fetuses increasing the risks of complications at delivery.
Early Delivery and Adverse Outcomes
The risk of delivery and newborn complications depends on the weeks of pregnancy. Delivering too early or too late can increase the risk.Based on a review of several studies of twin pregnancies published in 2016 in the British Medical Journal, the babies are higher risk for poor delivery outcomes including the following:
- Stillbirth of one or both twins
- Increased risk of death soon after delivery
- Respiratory distress because of underdeveloped lungs
- Newborn infection
- Needing to spend a longer time in the Neonatal Intensive Care Unit (NICU)
- Low Apgar scores and low birth weight for one or both newborns
- Increased risk of brain hemorrhage and other damage and cerebral palsy
- Increased risk of physical and mental developmental delays because of preterm delivery or low birth rate
Delayed Delivery of the Second Twin
When one twin is born prematurely because of preterm labor or preterm rupture of the membranes, the goal is to try to delay the delivery of the second twin as long as possible. There are no large studies on how long the second twin can survive in the uterus after the birth of the first, but case reports range from a few days to weeks.
One case report, for example, published in 2015 in Case Reports in Obstetrics and Gynecology, showed delivery of the second twin at 34 weeks after loss of the first at 17 weeks. This report, which includes a review of other case reports in the literature, reveals the following:
- The longer the second twin can stay undelivered the better the chance of survival.
- The duration and outcome depend on each person's situation and any complications that develop.
- After 32 weeks, the risk of complications appears to increase, including infection in the mother or second fetus and stillbirth.
- Delayed delivery is an option only if the second twin is in a separate amniotic sac from the first, does not share a placenta, and the sac is not ruptured.
There is no consensus on the best choices of treatments to delay delivery. In general, the focus is on preventing uterine contractions and infection and keeping the cervix closed to buy as much time as possible for the remaining baby to mature further.
Increase Your Chance of Delivering at Term
To improve your chance of carrying your twins to term:
- Keep all prenatal appointments and get all recommended ultrasounds and fetal monitoring. Expect to have more frequent prenatal visits and ultrasounds than if you were carrying one baby.
- Eat a healthy pregnancy diet and drink enough water daily. You need to eat 600 extra calories a day and ensure that you take folic acid and iron as prescribed by your doctor.
- Engage in low impact exercise such as swimming and yoga, rather than jogging or other high impact aerobics.
- Find an OB specialist who has experience managing twin pregnancies.
- Follow you doctor's recommendations if he advises you to restrict work, travel, exercise or other physical activities, or orders bed rest for the remainder of your pregnancy.
- Be aware of signs and symptoms of labor. If you go into preterm labor, your doctor might give you steroids to mature your babies' lungs and medicines to try to stop your labor.
Routine bed rest has not been shown to improve the outcome if your pregnancy is uncomplicated, according to the most recent analysis of the Cochrane Database published in in 2017. It will likely be recommended if you or your babies have complications. The duration of bed rest, which might be in the hospital, depends on the reason for the recommendation and might last until delivery.
Close Monitoring of Your Twin Pregnancy
Carrying twins comes with special concerns beyond those of a singleton pregnancy. You have greater chance of having a preterm baby than carrying to term. Close monitoring by your OB doctor will keep track of the health and growth of your babies and the best timing of your delivery.