Fertilization

Fertilization usually occurs in the ampulla, the widened, highly ciliated segment of the uterine tube. This is where the oocyte and sperm meet, generally within 12–24 hours after ovulation. The tube is not merely a passageway: ciliary activity and tubal fluid help bring gametes together and support the earliest cleavage divisions. The clinically useful fertile window is not centred on a calendar date. It is the six-day interval made up of the five days before ovulation and ovulation day. Sperm deposited before ovulation can remain capable of fertilization; in prospective hormone-confirmed data, conception rose from about 10% after intercourse five days before ovulation to about 33% on ovulation day. Intercourse after ovulation day did not result in conception in that study. A person who ovulates on cycle day 14 may be typical in a 28-day cycle, but day 14 is not a reliable prediction for an individual cycle. The follicular phase varies substantially, whereas the luteal phase is relatively more stable. Ovulation is therefore better estimated by counting backward from the next menstrual period or by using measured signs, such...