Conception Calculator —
fertile window and conception date.

Field guide

How conception dates are calculated.

Estimating the date of conception requires understanding two distinct biological measurements that obstetricians use interchangeably: gestational age (counted from the first day of the last menstrual period, LMP) and embryonic age (counted from actual fertilisation). Gestational age is always approximately two weeks longer than embryonic age, because ovulation and therefore fertilisation — typically occurs 14 days after the LMP.

The two calculation methods

The 266-day figure is the average embryonic duration of a human pregnancy (fertilisation to birth). The 280-day figure (40 weeks) is the gestational duration — 266 + 14. The standard-cycle assumption is 28 days; for every day your cycle differs from 28, the LMP and due date shift by the same amount while the 266-day conception-to-birth relationship remains constant.

The fertile window: why conception happens over days, not minutes

A common misconception is that conception requires sex on the exact day of ovulation. In practice, pregnancy can result from intercourse on any of the 6 days ending on ovulation day. This is because:

• Sperm survival: Healthy sperm can survive in the female reproductive tract for 3–5 days under favorable cervical mucus conditions. Sperm deposited 5 days before ovulation can still fertilize an egg.
• Egg viability: Once the ovarian follicle releases the oocyte, the egg is fertilizable for approximately 12–24 hours. After that window closes, fertilization is not possible until the next cycle.

The result: a roughly 6-day fertile window ending on ovulation day, where the probability of conception from a single act of intercourse rises through the window and peaks on ovulation day itself.

Daily conception probabilities (research-based)

Research published by Wilcox et al. (1995) in the New England Journal of Medicine, later refined by Dunson et al. (2002), established the relative probability of conception for each day of the fertile window from a single act of intercourse:

Note that day −1 (the day before ovulation) actually shows higher average pregnancy probability than ovulation day itself in some studies. This is partly because timing intercourse so that sperm are already present when ovulation occurs can be more effective than relying on post-ovulation intercourse.

How cycle length affects the conception date

Ovulation timing is often expressed as occurring 14 days before the next expected period — not 14 days after the LMP. This distinction matters for non-28-day cycles:

• 28-day cycle: ovulation typically around day 14 from LMP
• 35-day cycle: ovulation typically around day 21 from LMP (35 − 14 = 21)
• 21-day cycle: ovulation typically around day 7 from LMP (21 − 14 = 7)

The luteal phase, the time from ovulation to the next period, is relatively consistent at 12–16 days for most women regardless of cycle length. The follicular phase (LMP to ovulation) is the variable part. This is why the calculator asks for your cycle length.

How accurate is the estimated conception date?

The estimated conception date is exactly that — an estimate with a range of uncertainty:

• LMP method uncertainty: Ovulation can vary by ±2–3 days even in women with consistent cycles. Stress, illness, and travel can shift it further. The 3-day “most likely” window in this calculator represents one standard deviation around the mean; the full fertile window encompasses nearly all realistic outcomes.
• Due date method uncertainty: If the due date came from an early ultrasound (before 14 weeks), it is typically accurate to within 5–7 days. Due dates set later in pregnancy carry more uncertainty. Working backwards from the EDD carries all the same uncertainty in the forward direction.
• Irregular cycles: For women with irregular cycles or conditions such as PCOS, ovulation timing is less predictable. In these cases, an early dating ultrasound is the most reliable method for confirming conception timing.

Conception and gestational age in obstetrics

Clinicians almost universally use gestational age(from LMP) rather than embryonic age (from conception). This means:

• At the time of a positive pregnancy test (typically 10–14 days after conception), the gestational age is already 3.5–5 weeks.
• The “first trimester” ends at week 13 of gestational age, which is only 11 weeks after conception.
• The due date is always expressed as 40 gestational weeks — most babies arrive between 38 and 42 weeks.

This calculator reports gestational age (the clinical standard) in its “How far along” display, which will always show approximately 2 weeks more than the embryonic age counted from the conception date.