What You Need to Know About Twin Pregnancy

Twin pregnancy is considered a multiple gestation pregnancy. It can occur if more than one egg is released, and both are fertilized by a sperm. This is the case of fraternal twins, which are more common than identical twins. Identical twins occur if the fertilized embryo splits, then implants in the uterus. Fertility drugs and older maternal age are two risk factors for twinning. There may be two placentas, called dichorionic twins (DC), or one placenta, called monochorionic twins (MC).

Key takeaways:
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    Twin pregnancy can occur if more than one egg is released, and both fertilized by a sperm. This is the case of fraternal twins, which are more common than identical twins.
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    Identical twins occur if the fertilized embryo splits, then implants in the uterus.
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    Mothers of twins face increased risks such as hypertensive disorders, primarily preeclampsia, and gestational diabetes.
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    Work with your healthcare team to ensure a healthy pregnancy and successful birth.

There are three types of twins based on chorions (outer membrane) and amnions (inner membranes:

  • Dichorionic-diamniotic — Twins who have their own chorions and amniotic sacs. They typically do not share a placenta and can be fraternal or identical.
  • Monochorionic-diamniotic — Twins who share a chorion but have separate amniotic sacs. They share a placenta and are identical.
  • Monochorionic-monoamniotic — Twins who share one chorion and one amniotic sac. They share a placenta and are identical.

Causes of twins and risks

Even though assisted reproductive technologies have increased, the number of twin pregnancies has declined since 2014.

There are many risks associated with twin pregnancies that include premature birth, cerebral palsy, growth restriction, stillbirth, and death after birth. Prematurity happens 50% of the time, either due to contractions that will not stop or because one of the babies is in jeopardy. They also face unique risks that only twins can experience, described below.

Mothers of twins face increased risks such as hypertensive disorders, primarily preeclampsia, and gestational diabetes. Women also may experience worse breast tenderness and nausea.

Prenatal screening for chromosomal abnormalities

Screening for chromosomal abnormalities in twins can be done with non-invasive prenatal testing (NIPT) or by a combined test that includes maternal age, nuchal (back of the neck) translucency, serum pregnancy-associated plasma protein-A level, and β-human chorionic gonadotropin level.

Some doctors favor using maternal age and nuchal-translucency measurement alone. NIPT measures fetal cell-free DNA in the maternal circulation.

Maternal changes in twins

Nutritional needs for women pregnant with twins increase from 300 extra calories per day to 600 extra calories. Exercise of low to moderate intensity is recommended but if premature contractions occur, this will need to stop, and bed rest is often needed.

Assessment of fetal growth

Twins generally have lower birth weights compared to a single baby. Ultrasound is usually the first to diagnose twins and sequential ultrasounds are performed throughout pregnancy due to the fact that the twins may be growing at a slower pace due to crowded conditions and having to share nutrients from their mother.

It is recommended that twin growth charts be used rather than comparing their growths to singleton babies, which tend to be bigger in size. This helps to reduce the number of twins diagnosed as too small for gestational age or fetal growth restriction (FGR).

Monochorionic (MC) twins tend to have FGR, and higher perinatal mortality rates compared to dichorionic (DC) twins.

The recommendation for ultrasounds varies depending on MC versus DC, with DC twins receiving them every four weeks and MC every two weeks. One twin or both twins can be diagnosed with FGR. Depending on the discrepancy between the two weights, management may differ, and risks are higher if the discrepancy is significant.

Fetal Doppler measurements (blood flow in various umbilical/placental vessels) allow for detection of placental insufficiency and twin anemia–polycythemia (excess platelets) sequence (TAPS) and fetal peril in twin–twin transfusion syndrome (TTTS) and FGR.

Management of fetal growth restriction

Multiple modalities are used to evaluate each twin’s risk due to FGR. Early delivery may be recommended if there are connecting blood vessels in the placentas whereby one twin can send its blood to the other twin, as in twin-twin transfusion or if one baby dies in utero. This scenario is even more dire, as dead fetal tissue can be sent to the surviving twin.

Due to the higher risks of preterm delivery (68%), intrauterine death (15%) and neurological complications (26%) in the surviving twin in the case that the FGR twin dies, prenatal intervention may be necessary in the form of selective termination or laser therapy to block the vessels that are connected. In more severe cases before 26 weeks, termination is more likely to be recommended.

Timing of delivery

After 26 weeks gestation, early delivery can be considered in cases of severe FGR of one twin because stillbirth can occur in the smaller twin. Steroids are given two days before the delivery to enhance lung maturity. Every case is different and in DC twins, it is best to wait until after 30 weeks if one twin has severe FGR.

If both twins are growing normally, then waiting for spontaneous labor or scheduled cesarean around 36 to 37 weeks is appropriate.

The chance of cesarean birth is higher with twins. Sometimes, twins can be delivered by vaginal birth. It depends on:

  • The number of babies, their positions, weights, amniotic fluid, and health. If both babies are head first, that increases the chance of a vaginal birth.
  • Your health and how your labor is going.
  • The experience of your obstetrician-gynecologist.

Management of Twin-Twin Transfusion Syndrome

The diagnosis of Twin-Twin Transfusion Syndrome (TTTS) is based on amniotic fluid levels that differ significantly between the two twins. The risks to MC twins with TTTS are greater than for DC twins.

Intrauterine laser treatment can reduce this discrepancy in amniotic fluid levels and improve outcomes. In some cases, draining the fluid out of the twin’s sac with the high volume of amniotic fluid can improve outcomes if laser treatment cannot be done.

After the laser for TTTS, delivery is commonly scheduled around 34 weeks, due to research that the risk for perinatal death or severe brain injury is reduced significantly if delivery occurs after 34 weeks. But this is controversial and if otherwise, stable, delivery can be postponed until 36 to 37 weeks.

Screening and prevention of pre-eclampsia

By using a combination of maternal risk factors, serum biochemistry, mean arterial pressure and uterine artery Doppler, the risk for developing preterm pre-eclampsia can be measured. In singleton patients, aspirin can reduce the rate of pre-eclampsia, but no studies have been done on twins. However, most doctors recommend aspirin in twin pregnancies.

Screening, prevention and management of preterm birth

Twin pregnancies are at high risk for preterm delivery compared to single babies. Half deliver before 37 weeks and 15% prior to 34 weeks. Cervical-length screening by transvaginal ultrasound measurements may be a good assessment tool in asymptomatic women. However, the role of cerclage (procedure where sutures are used to sew the cervix shut) for a short and dilated cervix is debatable.

Maternal corticosteroids can reduce the rates of perinatal death, respiratory distress syndrome (RDS), intraventricular hemorrhage and necrotizing enterocolitis in those with preterm birth before 34 weeks.

There are many risks associated with twin pregnancies, for both the twins and their mother, who may experience more pronounced symptoms than if pregnant only with a single baby. If you are pregnant with twins, get regular prenatal care, ideally with an obstetrician-gynecologist.


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