Fetal Development | Fetal Development With Twins

Dr. Lillian Schapiro, an OB/GYN and women’s health expert with Peachtree Women’s Specialists, P.C. in Atlanta, Ga., knows all about carrying twins. After five cycles of fertility treatments, she became pregnant with twin daughters and delivered them in 1996. Her experiences inspired her self-published novel, Tick Tock (iUniverse, 2005), where the main character, a female gynecologist in the midst of a fertility rotation, discovers she may be infertile herself. She gives us a look at what really goes on in the womb during the development of twins.

Most women discover they are expecting twins during the first ultrasound when two yolk sacs become visible.

First Trimester: Formation of Embryos

Fraternal twins, which are the result of two separate fertilization events, are genetic and also the result of in vitro fertilization and assisted reproduction, Dr. Schapiro says. Identical twins occur naturally during conception and are formed from a single embryo whose cells dissociated from each other.

Most women discover they are expecting twins during the first ultrasound when two yolk sacs become visible. From there, several different things can happen as the embryos develop, Dr. Schapiro says. While identical twins usually grow within separate amniotic sacs, they sometimes share the same placenta. Fraternal twins have separate placentas and amniotic sacs.

Dr. Schapiro says that sometimes identical twins form in the same sac, but that can prove to be risky for the fetuses, as the blood supply from the placenta may travel more to one baby over another. If twins share the same sac it could also lead to the umbilical cords getting tangled as well.

During this important time of development, moms are encouraged to take in an extra 600 calories each day from healthy foods with plenty of calcium, protein, whole grains, fruits and vegetables. Only one prenatal vitamin is needed daily, as too much vitamin A could lead to heart defects in the babies.

About seven weeks into the pregnancy with twins, the babies’ genitalia begin to form, and about a week after that, the eyes, ears, mouth, lips, tongue, arms, fingers, knees and toes have formed as well, Dr. Schapiro says. At around 11 weeks, the twin scan be called fetuses, and each are about 4 inches long at this point.

Second Trimester: Growing Times Two

The babies will begin coordinated movement around 14 weeks, and the placentas (or placenta) have finished forming and are fully functional, with simple sugars, protein, fat, water, vitamins and minerals passing through them.

Not surprisingly, a woman carrying twins is going to measure larger than a woman carrying a singleton throughout the pregnancy. Around 20 weeks, the babies will begin to develop hair on their head, eyebrows, fingernails and toenails. In a healthy pregnancy, they will continue to gain weight and length at this point.

Most women carrying twins have a perinatologist, or maternal fetal medicine specialist, as their primary physician during the pregnancy. Multiple pregnancies are monitored a little more closely than single pregnancies to make sure the babies are experiencing equal growth and blood flow in the womb.

Third Trimester: Lots of Rest for Lung Development

By 28 weeks of a twin pregnancy, an expectant mom will probably be measuring full-term. This is a time where a woman should get plenty of rest and take precautions to prevent preterm labor, because the size of the uterus could trick the body into believing it’s time for labor to begin, Dr. Schapiro says.

Around 30 to 32 weeks, your doctor will want to monitor the growth of the baby every two weeks. Twins are generally considered mature enough for delivery around 37 to 38 weeks.

“If a woman carrying twins starts experiencing preterm labor, she may be given steroids to help the babies’ lungs develop before 34 weeks,” Dr. Schapiro says. “After that, doctors generally don’t administer steroids, as the lungs are probably developed enough for delivery.”

Twin placentas generally mature more quickly, and many twins are delivered either vaginally or via C-section around 38 weeks, depending on the positioning of the babies, Dr. Schapiro says. During the third trimester it’s hard to anticipate when a woman might go into labor, as many factors can come into play.

Pam Freiberg’s twins were due in mid December, but she was induced a few days before that because both her girls were already fully developed in the womb and she wanted to be home by Christmas with her other child at home. Both girls were delivered vaginally and each weighed 7 pounds.

“Both were absolutely perfect,” says Freiberg, a Chesterfield, Mo., resident. “The second one needed oxygen at first but was fine. Not exactly what I expected when the multiples were discovered at 18 weeks.”

Allison Yrungaray from Riverside, Calif., on the other hand, was completely surprised when her water broke at 35 weeks. The second baby delivered, her son, was slightly smaller and had to be in the NICU for three weeks following the birth.

“I would have gladly gone on bed rest beforehand to avoid having one twin in the NICU and the other one at home,” Yrungaray says. “That was hard.”

A Double Developmental Timeline

A multiple pregnancy is a miraculous thing for a mother to experience, but for the most part, multiple embryos develop pretty much on the same timeline as a singleton pregnancy, except the womb has an extra embryo to protect and nourish. After the implantation, most of the time the fetuses will grow and develop fairly equally and the size of the uterus will let the body know when it’s time for labor to begin.

Carrying multiples places extra demands on the mother, no doubt, but for the most part she can rest assured that each baby is developing as any other fetus would, just with an extra partner to share the experience with in the womb.