Finland researchers found that 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).
On the flip side, embryos that spend long periods in culture are less likely to be born small for their gestational age, MyHealthNewsDaily reported.
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.
Still, additional, larger studies are needed to confirm the new findings, the researchers said.
In the study, researchers at the University of Helsinki analysed information from 1,079 singleton babies (not twins) who were born after their mothers had undergone IVF.
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During IVF, eggs from the mother are fertilised 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 per cent of babies are born small for gestational age, 10 per cent are large for gestational age, and 80 per cent are normal weight. The average weight of babies in the study was about 3.5 kg.
Among embryos that were cultured for two to three days, about 10 per cent were small for gestational age, and 10 per cent were large for gestational age, as expected.
But among those that spent five to six days in culture, close to 19 per cent were large for gestational age, and 3 per cent 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, found the study published in the journal Human Reproduction.