Pregnancy · Due Date
How to Calculate Your Pregnancy Due Date (Naegele's Rule Explained)
Learn how due dates are calculated from your last menstrual period, how accurate they are, when ultrasound may change your date, and what to do if you do not know your LMP.
Updated July 7, 2026
Pregnancy & medication disclaimer
Medication safety in pregnancy is highly individual. Do not start or stop any drug based on this article alone. Contact your obstetrician, midwife, or pharmacist before taking any prescription, over-the-counter medicine, or supplement during pregnancy or while trying to conceive. In an emergency, call 911 or your local emergency number.
A positive pregnancy test often leads to the same question: when is my due date? Healthcare providers usually calculate it from the first day of your last menstrual period (LMP), not from the day you conceived. That can feel confusing — here is why, and how to estimate yours.
Naegele's rule (the standard formula)
The classic method — called Naegele's rule — works like this:
- Take the first day of your last period
- Add 7 days
- Subtract 3 months (or add 9 months)
- Add 1 year if needed
Mathematically, that is 280 days or 40 weeks from LMP. Pregnancy is dated this way because most people know when their period started more reliably than when conception happened. Conception usually occurs about 2 weeks after LMP in a 28-day cycle.
Skip the math: use our free due date calculator for an instant estimate plus trimester and week-by-week timeline.
How accurate is your due date?
Treat a due date as a rough target, not a deadline. Normal full-term birth can happen anywhere from about 37 to 42 weeks. Only a small fraction of babies arrive on the exact calculated date.
If your cycles are irregular, longer than 35 days, or you were on hormonal birth control recently, an LMP-based date may be off by a week or more. A first-trimester ultrasound is usually the most accurate way to date a pregnancy when LMP is uncertain.
When your due date might change
Your provider may adjust your official due date after an early ultrasound if fetal measurements differ significantly from LMP dating. Always use the date your prenatal care team gives you for clinical decisions.
What if you do not know your last period?
If you are unsure of LMP, do not rely only on symptoms or the date of a positive test. Schedule prenatal care — an ultrasound and clinical history can estimate how far along you are.
Pair with ovulation tracking when planning
If you are still trying to conceive, pair due-date planning with our ovulation calculator to understand your fertile window first.
Pregnancy & medication disclaimer
Medication safety in pregnancy is highly individual. Do not start or stop any drug based on this article alone. Contact your obstetrician, midwife, or pharmacist before taking any prescription, over-the-counter medicine, or supplement during pregnancy or while trying to conceive. In an emergency, call 911 or your local emergency number.