On average, pregnancy in humans lasts 38 weeks from conception.

Usually ovulation and conception occurs 2 weeks before the next period.  If periods are regular, the time of conception may be predicted as 2 weeks back from the next period in anticipation.

For ladies whose periods occur once 4 weeks, ovulation occurs 2 weeks from the last menstrual period (LMP), and the expected date of confinement (EDC) is then 40 weeks from the LMP.

Alternatively, we calculate the EDC for ladies with regular 4-weekly cycles as LMP + 9 months + 7 days.  Ladies with long cycles have EDC corrected according to cycle length.

Nowadays pregnancy tests are positive at about 2 weeks from conception, and it is about time to expect the ‘missed period’.

Many ladies like to quote their own gestation according to completed months.  Actually each month averages 4 1/3 weeks.  Therefore conversion between layman terms (months) and doctor terms (weeks) is 4 1/3 times number of completed months from LMP, plus the number of days between present one and the day of LMP.