Obesity Paradox #17 — Fat and risks for premature babies

The first report of the new year in the Obesity Paradox series is a just-published study that adds important information to our understanding of why fat pregnant women have lower risks for spontaneously delivering premature babies. The fact that doctors have known that fat women are more likely to carry babies to term may even be news to some. This research has received little news coverage, but it is information that women and all mothers-to-be deserve to hear.

This study was conducted by doctors with the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) Maternal–Fetal Medicine Units Network (MFMU), a network of fourteen university medical centers that have been involved in clinical research in improving outcomes for mothers and babies, with a focus on preterm births, for more than two decades. This study was just published in Obstetrics & Gynecology, the journal of the American College of Obstetricians and Gynecologists, with lead author Dr. Hugh M. Ehrenberg, M.D.

The authors began with a brief overview of what is known about weight and risks for delivering premature babies, writing:

Low pregravid BMI and inadequate weight gain are independent risk factors for preterm birth. Change in BMI between pregnancies may alter the risk of recurrent preterm birth, in that weight loss after a preterm delivery may increase the risk of recurrence. Conversely, the risk of spontaneous preterm birth decreases with increasing maternal BMI. The mechanisms responsible for reduced spontaneous preterm birth among obese/over weight women are not known. One plausible explanation is a reduction in uterine activity in these women.

The first three sentences, alone, may be surprising, especially given the continual media drumbeat about the dangers of obesity and weight gain! As we’ve observed, RWJF-supported anti-obesity initiatives have been promoting weight loss before and between pregnancies, and it had commissioned the Institute of Medicine to propose changing the clinical guidelines to recommend more restrictive weight gain during pregnancy. This 22-month initiative has been previously reviewed, along with the reasons why the weight guidelines are as they are today. Public health officials had previously reversed advice to restrict weight gain during pregnancy, after realizing by the 1960s that it was ill-founded and putting babies at risk and resulting in poorer survival.

So, before we look at this latest study, let’s catch up on the background research, including the studies referenced by these doctors in their introduction.

Why premature births matter. Reducing preterm deliveries is important to everyone who cares about the survival, health and futures of babies.

As we’ve covered, prematurity is the leading cause of infant death, accounting for two-thirds of babies in the United States who die in the first month of life. While we can’t forget that the overall infant mortality in our country has nearly halved over the past 15 years, largely due to wonderful advancements in neonatal intensive care, these small babies remain much more vulnerable to long-term health problems, such as cerebral palsy, mental retardation, chronic lung disease, vision and hearing loss, learning difficulties, and poor growth and development. Premature births also appear to raise risks for earlier onset of chronic diseases later in life, such as heart disease, hypertension and diabetes. Trying to prevent as many needless premature deliveries really does matter.

Among the things known to play a role in prematurity, as we’ve looked at, include maternal age, multiple gestation, low weight in the mother and low weight gain during pregnancy. During the 1950s, recommendations to restrict maternal weight gain during pregnancy were resulting in smaller babies with dangerously more health complications and poorer chances for survival. The risks for having a baby with intrauterine growth retardation were shown to double among women not gaining adequate weight.

As Dr. Ehrenberg and colleagues earlier reported in 2003, among the deliveries at their center between 1997 and June 2001, low pre-pregnancy weight and low pregnancy weight gain (less than 24 pounds) were each associated with higher incidences of low birth weight babies, premature babies and delivery complications in the mothers, even compared to women weighing 200 pounds. More recently, there have been growing concerns among healthcare professionals that today’s obsessions with thinness among women of childbearing age is heightening pregnancy risks. The recent surge in underweight babies being born in British Columbia spurred health officials there to tell mothers that it’s not what they eat, but how much that’s important to having a healthy baby.

The National Institute of Child Health and Human Development (NICHD) Maternal–Fetal Medicine Units Network (MFMU) had recognized well over a decade ago that preterm deliveries are responsible for about 8 out of 10 perinatal deaths in the United States. “Spontaneous preterm birth because of labor and premature rupture of the membranes accounts for approximately two-thirds of prematurity,” they reported in a 1996 issue of the American Journal of Obstetrics and Gynecology. While the importance of reducing spontaneous preterm deliveries were realized, they were unable to develop a risk assessment tool that was sufficiently predictive to be valuable for clinical use. The complex reasons for premature deliveries haven’t been sorted out.

Studies looking at associations between weight and preterm deliveries have frequently been complicated by unaccounted socioeconomic factors, they reported. In their 1996 analysis, the NICHD had demonstrated that when confounders were controlled for, the risks of spontaneous preterm births decreased with increasing BMIs in the mothers.

As we’ve covered, the 2007 study by doctors in the Department of Obstetrics and Gynecology at Drexel University in Philadelphia, PA, had reported that “maternal thinness is a strong predictor of both preterm birth and fetal growth restriction.” Rather than a retrospective data dredge, they had prospectively followed more than 3,000 pregnant women beginning at 23-24 weeks gestation until delivery, performing in-depth medical assessments and monitoring numerous potential risk factors in detail.

According to authors from the Division of Reproductive Health at the Centers for Disease Control and Prevention, at least a dozen studies published prior to 2000 had documented the association between higher preterm deliveries and both low prepregnancy BMI and low weight gain during pregnancy. To parse out the possible underlying reasons for these observations, they analyzed the medical records for women in the 1988 National Maternal and Infant Health Survey. Their findings were published in Obstetrics & Gynecology. The weight women gain during pregnancy proved to be a significant factor in premature deliveries.

Compared to a 5.8% risk for preterm deliveries among women of average prepregnancy weight who gained an average amount of weight during their pregnancies, women with low prepregnancy BMI and who gained less than ½ pound/week had 2.4 times the incidences of premature delivery (14%). Women of average weight who had similarly low weight gain during pregnancy had more than a three-fold higher risk for preterm deliveries (19.6%).

The CDC researchers also found that fat women with BMIs in the ‘obese’ category and high weight gain (>1.5 pounds/week) were associated with the lowest risks for preterm deliveries of all (2.4%): less than half the risks seen among those of average weights and weight gain. While some believe fat women should gain less weight during pregnancy, they found that low pregnancy weight gain for obese women raised their risks for preterm deliveries to 9.3%.

Another point noted in this new study that may have come as a revelation was that researchers had previously shown that weight loss after a pregnancy can increase a woman’s risk of having a preterm delivery with her next pregnancy. The study they exampled had been published in a 2006 issue of the American Journal of Obstetrics and Gynecology and led by Dr. Amy Merlino, M.D., with Maternal-Fetal Medicine at the Department of Obstetrics and Gynecology at Case Western Reserve University in Cleveland, Ohio. These researchers had hoped to show that losing weight before getting pregnant, and bring a fat woman to a ‘normal’ weight, would reduce rates of preterm births, or at least not change them. Instead, after reviewing the medical records of 1,241 women who had delivered at their medical center between 1996 and June, 2004, they found that women who had spontaneously delivered a preemie during their first pregnancies significantly increased their risks of having a preterm delivery with their next pregnancy if they intentionally lost weight between pregnancies. While this study was small, it showed that the more weight the women lost between pregnancies, the greater the risks for subsequently having a preterm baby. A single BMI unit drop (about 6 pounds for a woman of average height) doubled her risks of having a spontaneous preterm delivery. Losing more than 5 kg/m² BMI was associated with nearly triple the risk of having a preemie (28% to 80%).

Recommendations that women lose weight to reduce their risks of a preterm or underweight baby has clearly not been supported by the evidence to date. In fact, as noted by Dr. Ehrenberg and colleagues, “obese women had fewer spontaneous preterm births” in an analysis of the PreTerm Prediction Study by the National Institute of Child Health and Human Development (NICHD) Maternal–Fetal Medicine Units Network at the NIH. It found ‘obese’ women had half the rates of spontaneous preterm deliveries of thin women. Spontaneous preterm deliveries were inversely related to prepregnancy weights. Over 16% of the deliveries among women with BMIs <19 were spontaneous preterm deliveries, compared to 11.3% among ‘normal’ weight women, and 8.1% and 7.1% among ‘overweight’ and ‘obese’ women, respectively. Obesity has repeatedly been shown protective against premature deliveries.

Which brings us to this month’s study of 253 women who had previously had a spontaneous preterm delivery or vaginal bleeding, and were at higher risk for having another preterm delivery. To investigate one possible explanation for lower spontaneous preterm deliveries among fat women, they studied uterine activity (contractions). Women with known abnormalities, such as placenta previa, major fetal anomaly, cervical cerclage in place, and on prophylactic tocolytic therapy, were not included in this study. The women were followed at eleven medical centers between 1994 and 1996. Twice daily, beginning at 22 weeks gestation, the women’s uterine activity was monitored at home (for 2 hours/day) and they were clinically examined every 1-2 weeks. At each medical exam, their cervical fluid was also tested and they had transvaginal ultrasound cervical length measurements (believed to help predict increased risks for preterm birth).

Overall, only 8.3% of the ‘overweight’ or ‘obese’ women had spontaneous preterm deliveries before 35 weeks, compared to 21.7% of the women with BMIs of 25 or less. The women with BMIs under 25 (average weight 129 pounds) had 2.63 to 2.9 times the risks for preterm deliveries between 22 and 32 weeks gestation, compared to the fat women (with average weights of 185 pounds).

There were also no statistical differences between the BMI groups and other known risk factors, such as fetal fibronectin levels, transvaginal cervical length or Bishop scores. “For each gestational age interval before 32 weeks, obese/overweight women had fewer mean contractions per hour and maximal contractions per hour than normal/underweight women,” they found. But after controlling for other risk factors, the linear associations between BMI and the frequency of contractions, and spontaneous preterm births didn’t hold, meaning, they said, the protective role of obesity was through a “mechanism other than uterine quiescence.”

For most of human history, fat has been life-sustaining and a sign of a woman’s ability to bear and nurture children. Obesity continues to be shown to have a protective relationship for carrying a baby to term in the soundest studies, as these researchers confirmed, but it was not explained by uterine activity.

The welfare of all babies and improving their chances for healthy futures should be the primary concern, not whether their mums are fat. All babies deserve a healthy start and all mothers-to-be deserve good prenatal care… and the very best information.

© 2009 Sandy Szwarc