Twin pregnancies are becoming more common because of the increased use of infertility medications and women having babies at an older age. Twins births now account for 3 percent of all deliveries in the United States with non-identical twins being more common than identical twins. A woman with a history of twins in her family has a higher chance of having twins herself, possibly involving inheritance of genes related to twinning. Obese and tall women are also more likely to have twins than underweight and short women.
An early ultrasound is the best method of confirming a twin pregnancy. This should be done between 6 and 10 weeks for the most accurate diagnosis. An early ultrasound is also important in determining if the twin pregnancy is identical or non-identical, if there is a membrane between the fetuses, and if there are any early signs of birth defects or miscarriage risk.
Types of twins
Twin pregnancies occur from two eggs being fertilized by two sperm or from a single embryo splitting into two embryos early in the gestation. When two eggs are fertilized by two sperm, the twins are called non-identical or “fraternal” because they are different from each other just like any other brothers or sisters. When a single embryo splits into two embryos the twin pregnancy is call identical because they share the same DNA and also look alike. It is important to determine the type of twin pregnancy by early ultrasound because they each have different risks and frequency of monitoring during the pregnancy.
You might hear your doctor talk about two “membranes” that are seen on ultrasound called the chorion and the amnion. Fraternal twins have two chorions and two amnions and are called dichorionic/diamniotic. Identical twins have only one chorion but can have one or two amnions; therefore, they are called monochorionic/monoamniotic or monochorionic/diamniotic.
Risks to the babies
Compared to singleton pregnancies, twin pregnancies are at higher risk of abnormal growth, birth defects and preterm delivery. The risks for identical twins are higher than for non-identical twins including miscarriage, birth defects, preterm birth, growth complications, and a condition called twin-twin transfusion syndrome. These complications increase the risk of fetal loss during the pregnancy and neurological problems after delivery.
Risks to the mom
Twin pregnancies also carry a higher risk to mother including morning sickness, anemia, preeclampsia (high blood pressure), gestational diabetes, intrahepatic cholestasis of pregnancy, and blood clots.
The recommended weight gain by term for women of normal weight carrying twins is 35 to 45 pounds. Specific recommendations for weight gain based on your starting weight should be discussed with your doctor. Studies have shown fewer preterm births in women who gain weight within these guidelines. Increased protein in the diet is essential for normal fetal and placental growth. Red meat, pork, poultry, fish and eggs provide high quality and quantity of protein and better iron absorption. A woman with twins of normal weight should increase her total calorie intake by 600 calories/day. Healthy weight gain after 20 weeks gestation is approximately 1.5 pounds per week.
Screening for birth defects and Down syndrome is available at 11 to 12 weeks gestation with a test called the First Trimester Screen. This includes an ultrasound assessment of the fluid behind the neck of the fetuses called “nuchal translucency” and a blood test. The First Trimester Screen can identify 75 to 85 percent of pregnancies with Down syndrome and 67 percent of pregnancies with trisomy 18. The nuchal translucency measurement can also help identify twins at high risk for twin-twin transfusion syndrome.
Most twin pregnancies should be followed closely by ultrasound. After an early ultrasound to determine the type of twins and location of membranes, serial ultrasounds are usually begun at 16 weeks gestation. Birth defects are three to five times more common in twins and therefore a detailed anatomy ultrasound is performed at 18 to 22 weeks gestation. Non-identical twins are usually followed with ultrasound every four weeks throughout the pregnancy whereas identical twins are followed with ultrasound every two weeks beginning at 16 weeks gestation because of the special risk of twin-twin transfusion syndrome. At each ultrasound exam, the fetal growth and amniotic fluid volume are evaluated with additional testing ordered if problems occur.
Tests for fetal well-being are called non-stress tests (NST) and biophysical profiles (BPP). There is no proven benefit of routine use of these tests in twin pregnancies unless there are other complications occurring like abnormal growth, too much or too little amniotic fluid, diabetes, high blood pressure, or certain birth defects.
Preterm birth is the most common complication in twin pregnancies. The average gestational age for delivery of all twins is 35 weeks and preterm birth occurs in 60 percent of patients before 37 weeks and 12 percent before 32 weeks. Preterm birth is highest when both twins are boys. Unfortunately, there are currently no effective methods of predicting or preventing preterm birth in twins. Methods used successfully to prevent early delivery in singleton pregnancies like progesterone supplements and placing a stitch in the cervix have not been shown to lower preterm birth in twin pregnancies. The most helpful thing you can do is notify your doctor if you have any signs or symptoms of preterm labor like bleeding, spotting, painful contractions, cramping or pressure.
The risk of fetal loss begins to increase in twins at 38 weeks gestation. Therefore, women with uncomplicated non-identical twins can undergo delivery at 37 to 38 weeks gestation and women with uncomplicated identical twins can undergo delivery between 34 and 37 weeks gestation. For twins complicated by abnormal fetal growth or amniotic fluid, birth defects, diabetes, or hypertension, delivery might occur at different times based on overall risk assessment. Vaginal delivery is recommended when both twins are head first at the time of labor. A C-section is recommended when twin A is breech. When twin A is head first and twin B is breech, the route of delivery will depend on the gestational age, size of the twins, and risks of breech delivery. Women with a history of a low transverse C-section can have a vaginal birth (VBAC) with twins if they want.