Time of implantation of the conceptus and loss of pregnancy

Background Implantation of the conceptus is a key step in pregnancy, but li ttle is known about the time of implantation or the relation between the ti me of implantation and the outcome of pregnancy. Methods We collected daily urine samples for up to six months from 221 wome n attempting to conceive after ceasing to use contraception. Ovulation was identified on the basis of the ratio of urinary estrogen metabolites to pro gesterone metabolites, which changes rapidly with luteinization of the ovar ian follicle, The time of implantation was defined by the appearance of cho rionic gonadotropin in maternal urine. Results There were 199 conceptions, for 95 percent of which (189) we had su fficient data for analysis. Of these 189 pregnancies, 141 (75 percent) last ed at least six weeks past the last menstrual period, and the remaining 48 pregnancies (25 percent) ended in early loss. Among the pregnancies that la sted 6 weeks or more, the first appearance of chorionic gonadotropin occurr ed 6 to 12 days after ovulation; 118 women (84 percent) had implantation on day 8, 9, or 10, The risk of early pregnancy loss increased with later imp lantation (P < 0.001), Among the 102 conceptuses that implanted by the nint h day, 13 percent ended in early loss. This proportion rose to 26 percent w ith implantation on day 10, to 52 percent on day 11, and to 82 percent afte r day 11. Conclusions In most successful human pregnancies, the conceptus implants 8 to 10 days after ovulation, The risk of early pregnancy loss increases with later implantation. (N Engl J Med 1999;340:1796-9,) (C) 1999, Massachusett s Medical Society.