TOXICITY OF PROGESTERONE TO MOUSE FETUSES.

Injection of 3.5 mg progesterone in 0.05 ml sesame oil into each of 2 amniotic sacs on the 15th day of pregnancy in mice resulted in 61 % mortality by day 18 in the fetuses in the injected sacs. In a second group of pregnant mice, intra-amniotic injection of oil without other treatment was associated with 13% mortality by the 18th day in fetuses in injected sacs. In a third group, intra-amniotic injection of oil alone and subcutaneous injection of the mother on days 15, 16 and 17 of pregnancy with 3.5 mg progesterone in oil/day was followed by 65% mortality by day 18 in the fetuses in the injected sacs. Since the mortality rate in the injected sacs was about the same whether progesterone was administered intra-amniotically or via the maternal circulation and the placenta, it appears that this hormone is directly toxic to the mouse fetus in late pregnancy. Fetal mortality on day 18 in 15 untreated control litters was 2%. {Endocrinology 75: 145, 1964) A DMINISTRATION of large amounts of J\. progesterone to the pregnant mouse has usually been followed by fetal death, particularly in the later stages of gestation (1-5). Similar treatment of pregnant rats resulted in an eclampsia-like condition in the mothers and death of the fetuses (6, 7). Progesterone was sufficiently toxic to newborn mice that the estimated LD50 was 0.1 mg/mouse on the first, 0.4 mg on the second, and 2.5 mg on the third neonatal day (4). Mouse blastulae in vitro succumbed when the concentration of the hormone in the culture medium was 8 /ug/ml or more (8, 9). The present experiments were undertaken to determine whether in the mouse progesterone causes fetal death indirectly through an effect on the mother or placenta or, as seemed more likely, by direct action on the fetus. Materials and Methods Female mice of the Brown Belt stock were mated and were inspected each morning for the presence of vaginal plugs. The day of finding a plug was counted as day 1 of pregnancy. A solution of 70 mg of progesterone/ml of sesame oil was prepared. Precipitation of the hormone was prevented by keeping the solution in an oven at 65 C. On day 15 of pregnancy the mice were partially anesthetized with a subcutaneous injection of sodium amytal. Anesthesia was completed by administration of ether, and the abdomen was opened by a mid ventral incision. The amniotic sacs were drawn into or through Received February 3, 1964. This research was supported by Grant HD00456-14 from the National Institute of Child Health and Human Development, USPHS. the incision, and their condition was noted. The progesterone solution, cooled just before use to 40 C or a little less, was then injected through a 27or 30-gauge needle into the 2 most cephalad amniotic sacs in 1 uterine horn. Great care was employed to avoid touching the fetus or tearing the sac during this process; leakage of oil or amniotic fluid was very slight or did not occur. When the injections had been completed, the amniotic sacs were returned to the abdominal cavity and the muscle and skin layers were sutured separately. The mothers tolerated the procedure very well. Each of 2 amniotic sacs in 48 mothers was injected once with 0.05 ml of solution (3.5 mg of progesterone). Each of 2 sacs in 49 control mothers was injected once with 0.05 ml of sesame oil only. In a third group of 20 mothers 2 amniotic sacs were injected once with 0.05 ml of oil only and each mother was injected subcutaneously with 0.10 ml of a 35 mg/ml sesame oil solution on days 15, 16 and 17 of pregnancy. Each mother thus received a total of 10.5 mg of hormone in subcutaneous sites. A fourth group of 15 pregnant mice served as untreated controls. All mice were autopsied on day 18 (Brown Belt mice usually deliver on day 20 or 21).