When Louis Muglia and a group of fellow researchers studied nearly 3,500 mothers and their babies in Finland, Denmark and Norway, they noticed a curious pattern: The data suggested that shorter mothers had shorter pregnancies, smaller babies and a higher risk for preterm births.
“The relatively shorter you were, the relatively shorter your pregnancy was,” said Muglia, director of the Center for Prevention of Preterm Birth at Cincinnati Children’s Hospital Medical Center. “This was manifested in each of the three populations.”
The differences were small — each increase of 1 centimeter in height translated to about 0.4 gestational days — but statistically significant. Muglia said the findings, published Tuesday in the journal PLOS Medicine, might eventually offer one clue in helping to combat a problem that affects millions of babies around the world each year.
The United States has one of the highest rates of preterm births of any resource-rich country. Preterm birth affects nearly a half-million U.S. babies each year, and related complications account for about a third of all infant deaths, according to the Centers for Disease Control and Prevention. Premature births also result in a variety of serious health problems, including vision loss and neurological disabilities.
Of course, a large body of research has shown that many elements can contribute to preterm births and affect the size and health of an infant; medical factors such as a mother’s weight and disease history, behavioral factors such as stress levels and tobacco or alcohol use, and environmental factors such as a lack of prenatal care and exposure to polluted air or drinking water can lead to preterm births. Serious gaps also exist between racial and ethnic groups; for instance, the infant mortality rate among black babies is 2.4 times higher than that of white infants, according to the CDC.
But Tuesday’s study suggests that genetic factors related to a mother’s height also shape the fetal environment and influence the length of a pregnancy.
"Our study suggests it is the mom's height itself that is helping to determine the length of gestation," Muglia said. "It's part of the equation."
The explanation for why height helps determine how long a pregnancy lasts remains unclear, he said. Some experts say height influences uterine and pelvic size, meaning babies inside of smaller mothers have less room to grow and develop. Another theory is that smaller women have a lower “basal metabolic rate” — the amount of energy needed to support the body’s basic functions while at rest — that limits the amount of nutrition they can provide to a developing fetus.
Whatever the case, doctors might do well to pay attention to the height of an expectant mother. Although we cannot change a person’s genetics, Muglia said, we can give them the best chance for delivering a healthy child.
“How a woman enters pregnancy is important,” he said, noting that shorter women in particular need to pay attention to maintaining proper weight gain, receiving adequate nutrition and getting regular medical care. “This is one more thing we need to think about as a woman starts into pregnancy.”
Tuesday's study was funded by the March of Dimes,a nonprofit foundation that aims to improve the health of mothers and babies. The group currently supports five research centers around the country dedicated to investigating the causes of premature births, an effort that includes teams of epidemiologists, geneticists, computer scientists and other experts.
“This new finding,” the group’s president, Jennifer L. Howse, said in a statement, “adds one small piece toward solving the much larger puzzle of preterm birth.”