Due date and ovulation

Due dates use Naegele's rule, the standard the NHS and most clinicians use. A pregnancy is dated as roughly 40 weeks (280 days) from the first day of your last menstrual period (LMP):

Due date = LMP + 280 days

From a known conception date, the count is 266 days (38 weeks), because ovulation is about 14 days after the LMP. For IVF the embryo age is already known, so the date is the transfer date plus 266 days minus the embryo age (3 or 5 days).

How many weeks pregnant you are now is counted from the LMP (or the equivalent LMP worked back from conception or transfer), as the NHS does.

An estimate only, not a medical diagnosis. Only about 1 in 20 babies arrive on the exact due date. Your midwife may revise the date after your dating scan. Editorially reviewed against NHS and gov.uk guidance.

Pregnancy is dated in weeks plus days from the first day of your last period, the way the NHS counts it. A full-term pregnancy is about 40 weeks (280 days).

Weeks pregnant = (today − LMP) ÷ 7

If you know your due date instead, the LMP is worked back as the due date minus 280 days. The first trimester runs to the end of week 12, the second from week 13 to 26, and the third from week 27 to birth.

Informational only, not medical advice. Your midwife dates your pregnancy precisely at your dating scan. Editorially reviewed against NHS and gov.uk guidance.

Ovulation usually happens about 14 days before your next period, because the luteal phase (from ovulation to the period) stays roughly constant while cycle length varies:

Ovulation day = cycle length − 14

The fertile window is the day of ovulation plus the 5 days before it, because sperm can survive in the body for up to about 5 days, as the NHS notes. Your next period is estimated as the first day of your last period plus your cycle length.

An estimate only. Cycles vary month to month, so this is a guide rather than a guarantee. Ovulation predictor kits or speaking to your GP can give more certainty.