Transfer of norethisterone(NET) and levonorgestrel(LNG) from a singie tablet into the infant's circulation through the mother's milk

A single tablet of either of the three different types of oral contraceptive preparations, viz. “Gynovlar” containing 3000 ug norethisterone (NET) and 50 ug ethinyl estradiol (EE₂) or “Ovral” containing 250 ug levonorgestrel (LNG) and 50 ug EE₂, or a daily progestogen only type — “Minipill” containing 30 ug of LNG only, were administered to 40 normal lactating women on a random basis. The sampling schedule in all the three body fluids, i.e. the maternal sera, breast milk and the infant's sera, was kept in such a manner that the peak levels of the contraceptive steroids would be expected to be present in these fluids. The results of this study indicate that the transfer ratio of LNG or NET from the maternal sera to her breast milk was approximately 10% (6–34%) for Gynovlar, 9% (5–18%) for Ovral and 6% (2–34%) for Minipill. However, it was interesting to observe that whereas the transfer ratio of NET or LNG from breast milk to infant's sera was similar for combination pills — 8% (3–23%) for Gynovlar and 12% (3–42%) for Ovral, it was significantly higher for progestogen only Minipills — 38% (13–92%) for LNG. The precise reason for the higher transfer ratio of LNG from breast milk to infant's serum in Minipill users cannot be explained.