Calculating Your Due Date: How the Naegele Rule Works
Few numbers occupy expectant parents as much as the due date. It marks the day the baby is calculated to arrive — and it is determined by a simple formula that has been in use for over 150 years: the Naegele rule, named after the Heidelberg obstetrician Franz Karl Naegele.
The fastest way to find your date is the <a href="/en/due-date-calculator">due date calculator</a>: enter the first day of your last period and you will instantly see your estimated due date, your current pregnancy week and the maternity-protection dates.
The formula in detail
The classic Naegele rule counts from the first day of the last menstrual period: take that date, add 280 days — that is 40 weeks — and you get the estimated due date. In its well-known short form the rule reads: first day of the last period, minus three months, plus one year and plus seven days.
The reason for the 280 days: because the exact moment of conception usually cannot be determined, but the first day of the period is well known, counting starts from that day. With a 28-day cycle there are roughly two weeks between the start of the period and ovulation — which is why, by this count, a pregnancy lasts 40 weeks rather than 38.
Why cycle length matters
The classic Naegele rule assumes an exact 28-day cycle with ovulation on day 14. Many women, however, have shorter or longer cycles. The extended Naegele rule accounts for this: for every day your cycle is longer than 28 days, the due date shifts one day later — and correspondingly earlier for shorter cycles.
An example: with a 32-day cycle the estimated due date moves four days later. The <a href="/en/due-date-calculator">due date calculator</a> applies this correction automatically as soon as you set your average cycle length.
Conception and IVF as an alternative
If you know the exact day of ovulation or conception — for instance through cycle tracking — the date can be determined even more directly: on average, 266 days pass from conception to birth. After IVF the timing is known exactly: starting from the day of embryo transfer, you count back according to the embryo's developmental stage, which again yields a precise date.
What the date really means
As precise as the formula sounds, the due date is an estimate, not a deadline. Only about 4% of babies are actually born on that exact day. The vast majority arrive in the two weeks before or after. A birth between the completed 37th and 42nd week is considered on time.
The decisive figure for the final dating is not the calculation anyway, but the ultrasound: in the first weeks the age of the embryo can be determined very accurately from its size, which is why the date set by the doctor may correct the Naegele estimate.
Conclusion
By simply adding 280 days, the Naegele rule provides a reliable first orientation for the due date — refined by your individual cycle length. But it does not replace a medical examination. Calculate your personal date without obligation in the <a href="/en/due-date-calculator">due date calculator</a> and discuss the result with your gynaecologist or midwife.