Cycle, conception, and dating a pregnancy
A typical menstrual cycle runs about 28 days (normal range roughly 21–35), divided into the follicular phase (menstruation to ovulation) and the luteal phase (ovulation to next menstruation). Ovulation usually occurs around 14 days before the next period, not necessarily on day 14. Because the egg survives roughly 12–24 hours after release and sperm can survive up to about 5 days in fertile cervical mucus, the fertile window is approximately 6 days — the five days before ovulation plus ovulation day itself. Conception is most likely in the 1–2 days around ovulation.
Once fertilization occurs, the embryo begins implantation about 6–10 days after ovulation. The developing placenta then produces human chorionic gonadotropin (hCG), the hormone detected by pregnancy tests. hCG roughly doubles every 48–72 hours in early viable pregnancy, then peaks around 8–11 weeks and declines — a pattern used clinically to assess early pregnancy viability (a slow rise can indicate a problem, though there is wide normal variation).
Dating a pregnancy relies on the last menstrual period (LMP). Naegele's rule estimates the due date (EDD): LMP + 7 days − 3 months + 1 year (with calendar/leap adjustments). This yields a gestational age of 40 weeks from LMP — about 2 weeks more than the actual time since conception (embryonic age), because ovulation occurs roughly two weeks after the start of the last period. Ultrasound dating in the first trimester is generally the most accurate method and often refines an LMP-based estimate by days to a week.
Gestational age is counted in weeks and days (e.g. 24+3 = 24 weeks, 3 days). Full term is a range, not a date: 37–42 weeks with sub-classifications (early term 37–38+6, full term 39–40+6, late term 41–41+6, postterm 42+). The due date is a statistical midpoint — only about 4% of births occur on the estimated due date; roughly half occur in the week before, and half in the week after.