New Zealand researchers found that sheep that were underfed 60 days before conception and 30 days after were much more likely to deliver their offspring early than their well-fed counterparts. Results of the study appear in the April 25 issue of Science.
"Relatively small changes in nutrient intake around the time of conception can have profound influences on the fetus many months down the track, altering fetal development and resulting in preterm birth," says study author Dr. Frank Bloomfield, a senior lecturer in neonatology at the University of Auckland in New Zealand.
"If these findings are indeed applicable to humans, then there would be significant implications for research into preterm birth, and in the future when more information is available, hopefully into preventative measures to reduce the incidence of preterm birth," he says.
Though as many as one in 10 babies are born prematurely, the cause of most of these early deliveries still remains a mystery. According to the March of Dimes, a preterm birth is one that occurs between week 20 and 37 of pregnancy. Babies born prematurely are more susceptible to numerous illnesses and face an increased risk of dying in infancy, according to the study.
Those at an increased risk of preterm delivery include women who have delivered early in the past, women younger than 17 and older than 35, women pregnant with more than one baby, smokers and women who regularly drink alcohol, according to the March of Dimes.
Bloomfield and his colleagues gave eight sheep less food to eat for two months before conception and for one month following conception. The food rations were decreased enough to cause the sheep to lose 15 percent of their body weight. For comparison, the study also included 10 normally fed pregnant sheep.
After the underfed sheep were pregnant for 30 days, their diets were returned to normal.
Late in the pregnancy, the researchers regularly measured fetal cortisol and adrenocorticotropin (ACTH), two hormones believed to play a role in triggering labor in sheep, and possibly in humans.
The normal gestation period for ewes is 145 to 150 days. Underfed sheep delivered their lambs about a week earlier than well-fed ewes.
The average length of pregnancy for the well-fed sheep was 146 days, compared to only 139 days for the underfed sheep. The lambs were all about the same size at birth.
The researchers saw a surge in fetal cortisol for only half of the underfed sheep, but all had higher levels of ACTH. The study's authors believe these findings suggest that undernutrition early in pregnancy results in an early maturation of the fetal adrenal gland, which then releases ACTH early and triggers premature birth.
Dr. Mark Dykowski, an obstetrician and gynecologist at William Beaumont Hospital in Royal Oak, Mich., isn't convinced these results will be similar in humans even though sheep provide a close model to human gestation. He says there are too many variables that affect human pregnancies, such as smoking, alcohol consumption, maternal age and prenatal care.
"It's an interesting area to look at, especially since it's a study that could never ethically be done in humans," says Dykowski. However, he adds, "there were only 18 sheep in the whole study. I don't know that you can make statistically significant comments with that sample size."
The bottom line, according to Dykowski, is that "good nutrition and appropriate weight gain are important for every pregnant woman."