Why we count from the last period
Ovulation and conception are invisible events. Before ultrasound existed, the only date a woman could reliably report was when her last period started. So medicine standardised on the first day of the last menstrual period (LMP) as “day 0” of pregnancy. It isn’t biologically the start – conception usually comes about 14 days later – but it gives everyone a shared starting line. Every scan chart, screening window and definition of term is built on it.
The side effect is the famous two-week head start: at the moment of conception, you are already considered 2 weeks pregnant, and when your period is missed you are about 4 weeks pregnant.
Naegele’s rule
Named after the German obstetrician Franz Naegele (1778–1851), the rule adds 280 days to the LMP. The pen-and-paper version is:
Example: LMP 15 March 2026 → minus 3 months = 15 December 2025 → plus 7 days = 22 December 2025 → plus 1 year = 22 December 2026.
Because months have different lengths, the paper version can be a day or two different from exact 280-day arithmetic. Our due date calculator always uses the exact day count.
Adjusting for cycle length
Naegele assumed a 28-day cycle with ovulation on day 14. But ovulation reliably happens about 14 days before the next period, so it drifts with cycle length. The modified rule (sometimes called Parikh’s formula in Indian textbooks) is:
With a 35-day cycle, add 7 days; with a 25-day cycle, subtract 3.
Other ways to date a pregnancy
- Conception date: conception + 266 days. Used when ovulation was tracked or after IUI.
- IVF: embryo transfer date + 266 − embryo age. See the IVF calculator.
- Ultrasound: the baby’s crown-rump length (CRL) in the first trimester is converted to gestational age using standard charts. Later, head circumference, abdominal circumference and femur length are used, with less precision.
- Physical examination: uterine size and fundal height give rough estimates but are no longer used for dating when a scan is available.
Which date wins?
If your LMP is certain and an early scan agrees within about a week, the LMP date is kept. If they disagree by more than set thresholds (5 days before 9 weeks, 7 days up to 16 weeks, and so on), your doctor will use the scan date. Once your due date has been set by a first-trimester scan, later scans are used to check growth – not to change the date.
After your due date is confirmed, use the Due date tab in any of our calculators so every result lines up with your doctor’s dates.
Frequently asked questions
Why is pregnancy 40 weeks and not 38?
Pregnancy is about 38 weeks from conception, but because we count from the last period (about 2 weeks earlier), it is described as 40 weeks.
Is Naegele’s rule accurate?
It’s a reasonable estimate for women with regular 28-day cycles. For others, adjusting for cycle length or using an early ultrasound is more accurate.
Medical disclaimer: The results and information on this page are estimates for general education only. They are not a diagnosis and do not replace advice from your gynaecologist, obstetrician or midwife. If you have pain, bleeding, reduced baby movements or feel unwell, contact your doctor or call 108 immediately. Read the full disclaimer.
Sources & further reading
- American College of Obstetricians and Gynecologists (ACOG). Committee Opinion No. 700: Methods for Estimating the Due Date. acog.org
- Mittendorf R, et al. The length of uncomplicated human gestation. Obstet Gynecol 1990;75:929–32.
- Jukic AM, et al. Length of human pregnancy and contributors to its natural variation. Hum Reprod 2013;28:2848–55.