Parenteral administration of progestin Nestorone® to lactating cynomolgus monkeys: an ideal hormonal contraceptive at lactation?

~40 mg NES/day in vitro, were placed s.c. in the mothers 3‐4 months following delivery, where they remained in situ for 4 weeks. Sampling (blood daily from the mother; milk and blood from the infant at 3 day intervals) was initiated at 2 weeks prior to insertion, and continued for 2 weeks following removal of the implant. NES, oestradiol, progesterone and prolactin were measured by radioimmunoassays and the infants were weighed weekly. The (mean K SD) maternal serum and milk concentrations of NES were 337 K 90 and 586 K 301 pmol/l during the use of the implants. The ratio of milk/serum NES was 1.68 K 0.12 (mean K SE), and the serum and milk concentrations were significantly correlated (r 5 0.75, P < 0.001). NES was not detectable (<13 pmol/l) in any infant serum samples. Concentrations of prolactin (mean K SD) were 41.1 K 32, 26.7 K 7.6 and 26.3 K 9.5 ng/ml before, during and after the use of the implants respectively. The (mean K SE) infant weight increased from 643 K 54 g 1 week prior to insertion to 713 K 54 g 1 week following removal. These data confirm that NES in milk is rapidly metabolized by the suckling infant. Therefore, NES appears to be an ideal hormonal contraceptive for use during lactation.