• Article
  • January 24, 2018

Placenta Affects Why One Twin is Smaller than the Other

A study finds that slower transport of oxygen from mother to baby across the placenta predicts slower fetal growth, as well as a smaller brain and liver.

When a baby is born small, it's often attributed to genetic factors or maternal risk factors like poor nutrition or smoking. But a twin study led by researchers at Boston Children's Hospital finds that slower transport of oxygen from mother to baby across the placenta predicts slower fetal growth. The study, published in Scientific Reports, is the first to make a connection between birth outcomes and placental oxygen transport.

By studying identical twins, researchers controlled for genetic factors and maternal risk factors. Although identical twins also share a placenta, it’s divided into two separate compartments, and one may be healthier than the other.

P. Ellen Grant, M.D., director of Boston Children's Fetal-Neonatal Neuroimaging and Developmental Science Center, and Elfar Adalsteinsson, Ph.D., at MIT developed a noninvasive method using MRI to map real-time delivery of oxygen across the placenta. Using this technique, called Blood-Oxygenation-Level-Dependent (BOLD) MRI, they showed that dysfunctional placentas have large regions with slow oxygen transport to the fetus.

"Until now, we had no way to look at regional placental function in vivo," Grant says. "Prenatal ultrasound or routine clinical MRI can assess placental structure, but cannot assess regional function, which is not uniform across the placenta."

Grant says Doppler ultrasound, the current clinical method of assessing placental function, measures blood flow in the umbilical arteries and other fetal vessels, but it cannot tell how well oxygen or nutrients are being transported from mother to fetus.

Real-time placental oxygen mapping

In the new study, part of the NIH-funded Human Placenta Project, researchers followed seven sets of identical twins to birth, tracking pregnancies in which one twin was smaller than the other. At 29 to 34 weeks of pregnancy, the seven mothers underwent BOLD MRI for about 30 minutes. 

While they inhaled pure oxygen for 10-minute stretches, Grant's team measured how long it took oxygen to reach its maximum concentration in the placenta, known as the time to plateau (TTP), and then how long it took for the oxygen to pass through the umbilical cord into the fetus and penetrate the brain and liver. 

They found that a longer TTP in the placenta correlated with lower liver and brain volumes and lower newborn birth weights. Grant hopes her team's work will be used to better understand pregnancy risk factors, develop a prenatal test for mothers in whom placental dysfunction is suspected and ultimately improve prenatal care.

"Our next goal is to figure out what causes variation in oxygen transport in the placenta and identify a cutoff value that would be of concern in a pregnancy, including singleton pregnancies," she says. "Then, we can think about treatments to improve placental oxygen transport, and use our methods to assess the success of these treatments."

Future research

Grant believes placental oxygen transport is a prime example of how environmental factors can modify the DNA we all inherit. Future studies will investigate how placental oxygen transport affects fetal gene expression and specific measures of brain development and organ metabolism.

"The placenta plays a key role in fetal development and maternal health," says David Weinberg, project lead for NIH's Human Placenta Project, launched by the Eunice Kennedy Shriver National Institute of Child Health and Human Development. "Understanding how it functions is essential for developing interventions to improve the health of mothers and infants."

Send questions or comments to magazine@childrenshospitals.org.