(CNN)Metformin use by men in the three-month period before they conceived a child was linked to a 40% higher risk of birth defects in the offspring, according to a study published Monday in the journal Annals of Internal Medicine.
Metformin is a first-line drug in the treatment of type 2 diabetes.
The research, which was done out of Denmark, used national registries to follow over 1 million births between 1997 and 2016 and compared the risk of major birth defects in babies based on paternal exposures to diabetes medications.
The study observed only children who were born to women under 35 and men under 40. Babies born to women with diabetes were excluded.
The researchers considered men exposed to metformin if they filled a prescription for it in the three months before conception, which is how long it takes the fertilizing sperm to fully mature.
According to the study, the frequency of birth defects in babies born to men who had a type 2 diabetes diagnosis but who were not taking metformin was 3.1% (1,594 children), but the frequency was 4.6% (788) in children with paternal exposure to metformin during the preconception period.
Genital birth defects, seen only in male babies, was the only birth defect in the study that was found to be associated with a statistically significant increased risk after paternal metformin use.
Men who took metformin before or after the three-month maturation period did not have an increased risk of having a baby with birth defects. Similarly, the babies’ unexposed siblings were not affected.
Because previous research has shown that diabetes can compromise sperm quality and impair male fertility, the researchers also compared the rates of birth defects in the babies of men who took insulin to those who took metformin …