Pregnancy & Fertility

Pregnancy length is traditionally counted from the first day of the last menstrual period and taken as 280 days (40 weeks) on average. Due date estimation and ovulation window calculation rest on this simple calendar logic.

Which tool for which question?

• Pregnancy Week and Due Date Calculator: Calculate your estimated due date (Naegele's rule), how many weeks pregnant you are, the days left and your trimester dates from the first day of your last period.
• Ovulation and Fertile Window Calculator: Estimate your ovulation day, fertile window (most fertile days) and next period from the first day of your last period and your cycle length.

How the calculation works

• The estimated due date comes from Naegele's rule and is adjusted for cycles other than 28 days; the gestational week comes from the days elapsed since the last period.
• Ovulation usually happens about 14 days before the next period, because the luteal phase after ovulation is more constant. Because sperm can survive for several days in the reproductive tract, the fertile window also covers days before ovulation.

Things to watch

• Results are estimates; the real birth date is often different from the calculated one.
• With irregular cycles the ovulation estimate can shift.
• Results do not replace medical advice, diagnosis or treatment; consult your doctor for health decisions.