Fertilization

Fertilization usually occurs in the ampulla, the wider, highly ciliated outer portion of the uterine tube. This is not the uterus. The tube captures the ovulated oocyte, supports sperm survival and transport, and is where the first cell divisions occur before the developing conceptus reaches the uterine cavity. Ovulation releases a secondary oocyte surrounded by cumulus cells and the zona pellucida. The oocyte remains capable of fertilization for only about 12–24 hours. Sperm can survive longer in favorable cervical and tubal conditions, which explains why intercourse before ovulation—not after it—is responsible for most conceptions. The fertile window is the five days before ovulation plus ovulation day. In prospective studies, conception occurred only when intercourse fell within this six-day interval. Probability is relatively low about five days before ovulation and rises sharply as ovulation approaches. Intercourse after ovulation is much less likely to result in conception because the oocyte’s fertile lifespan is brief. The LH surge is useful because it reflects a biologic event rather than a calendar guess: - Ovulation usually follows the start of the LH surge by about...