Babies conceived via fertility treatment may be slightly smaller at birth than newborns conceived naturally, a new study suggests.
In a recent study of 1,700 women who gave birth to a single baby each, researchers found that babies born to women with fertility problems weighed a little less -- about a third of a pound, on average.
They were also at somewhat greater risk of low birthweight (less than 5.5 pounds) for gestation.
The researchers can not quite explain the reasons behind this finding. But they believe that the underlying fertility problems may play a bigger role than the treatments themselves.
Couples who successfully undergo fertility treatment, including fertility drugs and in vitro fertilization (IVF), often end up with twins or more. But research has suggested that even when parents have a single baby, their babies tend to be smaller than newborns conceived naturally.
"But it's been hard to tease out, is it the infertility or the technology used to treat it?" said Dr. Amber R. Cooper, an assistant professor of obstetrics and gynecology at Washington University in St. Louis who led the new study.
So Cooper's team looked at records for 461 women who came to their center with fertility problems over 10 years and ultimately had a baby. More than half underwent IVF, while 106 were treated with fertility drugs, and 104 eventually became pregnant on their own.
Those women were compared with 1,246 fertile women who gave birth during the same time period.
Overall, babies born to women with fertility problems were smaller. But there was no difference in average birthweight between women who underwent IVF and infertile women who eventually had a baby without medical help.
The biggest gap was seen in the group of women who'd been treated with fertility drugs, which spur ovulation: Their newborns were about a half-pound lighter compared with fertile mothers.
On the other hand, the increased risk of low birthweight was mainly seen in IVF babies. Twelve percent of those moms had a low-birthweight newborn, versus just under eight percent of mothers with no fertility problems.
"The findings suggest that a large portion of this may be related to the underlying infertility," Cooper said.
Fertility drugs, for example, are often used for women whose infertility is tied to problems with ovulation -- like the hormone-related disorder polycystic ovary syndrome (PCOS). In this study, nearly half of the fertility-drug group had PCOS or some other ovulation disorder.
It's possible that those causes, rather than the drugs used to stimulate ovulation, could explain the lower average birthweight, according to Cooper's team.
IVF -- in which a woman's eggs and a man's sperm are joined in the lab dish -- may be done for different reasons. It's often the choice when a woman has a blockage in the fallopian tubes leading the egg to the uterus, for instance, or when the man has fertility problems. IVF may also be done only after fertility drugs or other less-extensive treatments fail.
Some past research has suggested that the longer a couple takes to conceive, the greater the odds of low birthweight or other complications. So the higher risk of low birthweight in IVF might be related to that, according to Cooper.
The researchers were not able to pin lower birthweights to any specific causes of infertility, though. The number of couples with any one diagnosis was too small to make reliable comparisons.
"I don't think we've ruled out that there could be some effect of the technology itself," Cooper said.
But, she added that fertility treatments are safe overall, and when couples successfully become pregnant, the outcomes are "overwhelmingly good."
As for the birthweight gap seen between fertile and infertile couples, it was relatively small but not unimportant, according to Cooper. "A half a pound is still fairly significant," she said.
Cooper noted that studies have linked poorer fetal growth and lower birthweight to a higher risk of certain health problems in adulthood, including high blood pressure and heart disease.
The so-called "fetal origins hypothesis" holds that impaired growth in the womb might help "program" a person's risk of certain health problems later in life.
Since close to two percent of U.S. births each year are now helped along with fertility treatment, Cooper said, it will be important to keep studying any effects of the treatments on long-term health.
"We're still working on making the technology the best that it can be," she said.