Levonorgestrel in milk and plasma of breast-feeding women with a levonorgestrel-releasing IUD.

A levonorgestrel-releasing intrauterine contraceptive device was used by 10 breast-feeding women beginning 6 weeks after delivery. Two models of IUD were used. One released 10 microgram (5 patients), the other 30 microgram (5 patients) of levonorgestrel per day. Plasma and milk samples were collected 8 times over a 3-month period and the concentrations of levonorgestrel determined by radioimmunoassay. An improved and sensitive method for the determination of levonorgestrel in milk was developed. A column chromatographic purification of a milk extract before radioimmunoassay made possible the use of large milk samples in order to improve the sensitivity. The plasma concentrations during the follow-up period were 207 +/- 64 pg/ml (mean + SD) in the 10 microgram and 235 +/- 87 pg/ml in the 30 microgram/day releasing IUD groups. The milk levonorgestrel concentrations were 56 +/- 35 and 57 +/- 34 pg/ml, respectively. The plasma to milk ratio of levonorgestrel was initially 100:15 and at the end of the 3-month follow-up period 100:25. There were no significant differences in the milk and plasma concentrations between the study groups. The total amount of levonorgestrel excreted per day in 600 ml breast milk is approximately 0.1 per cent of a daily dose of 30 microgram.