Here’s Why Shorter Moms May Have Shorter Pregnancies
A study finds that a mom's height directly influences how long she stays pregnant, leading to a better understanding of preterm birth and why it happens.
If you're small, it makes sense that your baby would be smaller too, right? August 2015 research published in the journal PLOS Medicine finds that not only do shorter women have babies with lower birth weights and lengths—but the duration of their pregnancy is actually shorter as well. This means that being shorter in stature may very well be a risk factor for preterm birth.
It’s Not Just in the Genes
Scientists at the March of Dimes Prematurity Research Center Ohio Collaborative studied over 3,000 women and found an interesting link between mothers' heights and babies who were born early. "As part of our genetic studies of factors that increase the risk for preterm birth, we collect other information about the mothers, such as their height, weight, and age," study author Dr. Louis Muglia, M.D., Ph.D., co-director of the Perinatal Institute at Cincinnati Children's Hospital Medical Center, says. "We found when we ran our analysis that mom's height was a risk factor for having a preterm birth, so we decided to investigate this further."
What the researchers discovered when they looked at the reasons behind this correlation was surprising. They found a genetic link from mom's height to baby's birth weight and length, which was to be expected—small moms make small babies. But, the pregnancy length was not simply caused by the mother's genetics, meaning something else was responsible for shorter women having shorter pregnancies.
"Whatever influences maternal height, such as mom's genes but also her nutrition and other health habits, influence how long she will carry her pregnancy," Muglia says. "We think this relationship may exist either because mother's height influences uterine size or pelvic size, or height is related to mom's metabolism and how much energy she can supply to a growing baby prior to birth."
In other words, environmental factors influence mothers' heights, which in turn influence how long baby can stay put. "Gestational length is controlled by more than the mother's genes, and whatever it was that makes the woman short also generates a shorter gestational length," says Joe Leigh Simpson, M.D., March of Dimes senior vice president for Research and Global Programs. "Presumably her height reflected other things that had gone on in the mother's lifetime—her nutritional status or exposure to pollution, for example."
What Can Short Moms Do?
There's not much you can do to change your genetics—but if you're on the shorter side, is there anything else you can do to ensure a full-term pregnancy? "I think height is just one of several factors that influence preterm birth risk," Muglia says. "Making sure to optimize other factors related to pregnancy health is key. These include being a healthy weight at the start of pregnancy, having appropriate pregnancy weight gain, not smoking, and waiting an appropriate time between pregnancies [18-23 months]."
Your doctor may also want to monitor you a bit more closely if you're very petite. "When I saw patients, with someone who is 4 feet 10 inches tall, you intuitively know that this pregnancy may not last as long and there may be a need for a cesarean section because of the size of the pelvis," Simpson says, "especially if the father is quite a bit bigger than the mother so the baby is bigger."
If you're a shorter mom, there's no need to be overly concerned about preterm birth, as long as you're taking proper prenatal care. But the study's findings are important because they are helping doctors get to the root of what causes preterm birth, and what actions can be taken to prevent it worldwide. Nothing can be done about genetics, but environmental factors like poor nutrition, which often restricts people's growth in low-income countries, can be improved. "We know that many women in very unfavorable situations—stress, pollution, poor nutrition—still deliver at full term; they just have a higher risk of not making it to full term," Simpson says. "Height is a part of the puzzle. But pregnancy is a very robust phenomena. Otherwise nobody who's short would ever have a full term pregnancy!"