The way in which embryos are prepared during in vitro fertilization may  influence the size of the baby that's born, a new study from Finland suggests.

Embryos that spend long periods growing in culture (around five to six days) before being transferred to the mother’s womb are more likely to be born heavier than normal for their gestational age, compared to embryos that spend a shorter period in culture (two to three days), the study found. (Gestational age refers to how far along a pregnancy is.)

On the flip side, embryos that spend long periods in culture are less likely to be born small for their gestational age, the researchers said.

Previous studies have shown babies born as a result of  IVF treatment are at an increased risk for preterm birth and low birth weight. Factors related to the pregnancy, or to the IVF technique itself, may be responsible for the association. Few studies have looked at the effect of culture time on the baby's birth weight, however.

Still, additional, larger studies are needed to confirm the new findings, the researchers said.

In the study, researchers at the University of Helsinki analyzed information from 1,079 singleton babies (not twins) who were born after their mothers had undergone IVF.

During IVF, eggs from the mother are fertilized in a laboratory, and allowed to grow in culture for about one to six days before they are transferred to the mother's uterus. Typically, embryos are transferred to the uterus after two to three days, according to the American Pregnancy Association.

The Helsinki researchers determined the percentage of babies that were born at a normal weight; that were small for gestational age; or that were large for gestational age. Generally, 10 percent of babies are born small for gestational age, 10 percent are large for gestational age, and 80 percent are normal weight.

The average weight of babies in the study was about 7.7 pounds.

Among embryos that were cultured for two to three days, about 10 percent were small for gestational age, and 10 percent were large for gestational age, as expected.

But among those that spent five to six days in culture, close to 19 percent were large for gestational age, and 3 percent small for gestational age, the researchers said.

Babies born small for gestational age are at increased risk for complications such as low blood sugar, and neurological disabilities. Later in life, babies that were small for their gestational age may be at increased risk for such chronic conditions as heart disease. Large-for-gestational-age babies may be heavier in childhood, and thus at increased risk for adult obesity, the researchers said.

The reason for the finding is not clear. It could be that certain stages of embryo development differ depending upon whether they take place in a laboratory or the womb, the researchers said.

Large-for-gestational-age babies were also more likely to be born to mothers with high body mass index or to women who had previously given birth. The mother's age, cause of infertility or method of fertilization did not affect the results.

The study was published online Dec. 11 in the journal Human Reproduction.

Pass it on: IVF embryos that spend long periods in culture may be at increased risk of being born large for gestational age.

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