One twin smaller than the other? The answer may lie in the placenta
Until now if a baby was born small it was generally attributed to genetics, or maternal risk factors such as poor nutrition or smoking.
However, a study of twin pregnancies has found another factor that can be measured prenatally: slower transport of oxygen from mother to baby across the placenta.
Published in Scientific Reports, the study is the first to make a direct connection between placental oxygen transport and birth outcomes. It relies on a new, non-invasive technique called blood-oxygenation-level dependent (BOLD) MRI. Developed by Dr Ellen Grant, director of the Fetal-Neonatal Neuroimaging and Developmental Science Center at Boston Children’s Hospital and Elfar Adalsteinsson, it maps oxygen delivery across the placenta in real time.
“Until now, we had no way to look at regional placental function in vivo,” said Dr Grant. “Prenatal ultrasound or routine clinical MRI can assess placental structure but cannot assess regional function, which is not uniform across the placenta. Doppler ultrasound can measure blood flow in the umbilical arteries and other foetal vessels, but it cannot tell how well oxygen or nutrients are being transported from mother to foetus.”
By studying identical twins, the researchers were uniquely able to control for both genetic factors and maternal risk factors. Although identical twins also share a placenta it is divided into two separate compartments, and one may be healthier than the other.
The researchers followed seven sets of identical twins all the way to birth. In all cases, 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 concentrations to peak in the placenta, known as the time to plateau (TTP). The team then tracked how long it took for the oxygen to pass through the umbilical cord into the foetus and penetrate the brain and liver.
To adjust for foetal motion, a team at MIT used image-correction algorithms.
The finding? A longer TTP in the placenta correlated with lower newborn birth weights as well as lower liver and brain volumes. TTP also correlated with placental pathology when placentas were examined after birth by pathologist Drucilla Roberts at Massachusetts General Hospital (MGH).
Grant hopes this work will lead to a better understanding of pregnancy risk factors and ultimately to a prenatal test for mothers in whom placental dysfunction is suspected.
“Our next goal is to figure out what causes variation in oxygen transport in the placenta and identify a cut-off value that would be of concern in a pregnancy, including singleton pregnancies,” she said. “Then we can think about potential treatments to improve placental oxygen transport, and use our methods to immediately assess the success of these treatments.”
Grant believes placental oxygen transport is a prime example of how environmental factors can modify our DNA. Future studies will investigate how placental oxygen transport affects foetal gene expression and specific measures of brain development and organ metabolism.
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