Pregnancy Due Date
Estimate your baby's due date.
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Forty weeks, counted from a date you might not expect
Most people assume pregnancy is timed from conception. It usually isn't. Doctors count from the first day of your last menstrual period, the LMP, because that date is far easier to pin down than the moment of conception. From that day, a full-term pregnancy runs about 40 weeks, and the estimated due date is simply 280 days later.
That's the heart of Naegele's rule, named after the German obstetrician who popularised it. It gives a target to plan around, for scans, leave from work, and that hospital bag by the door.
The shortcut your doctor uses
There's a neat mental version. Take the first day of your last period, add seven days, then count back three months. That's the same as adding 280 days, just easier to do in your head. Add a year if the subtraction pushes you into the previous calendar year.
Picture an LMP of 10 March 2026. Add seven days and you reach 17 March. Now go back three months, from March to December, which lands you in the previous year, so add one to keep it forward: 17 December 2026. That's the estimated due date. Roughly 40 weeks of growing, mapped from a single date.
Splitting the journey into three
Those 40 weeks divide into three trimesters, each a distinct stretch. The first runs through about week 12, when the major organs form and morning sickness often peaks. The second, roughly weeks 13 to 27, tends to feel easiest, with energy back and the first kicks arriving. The third carries from week 28 to birth, when the baby gains weight fast and the waiting feels longest.
Knowing which trimester you're in helps you read what your body is doing and what each checkup is looking for.
A date to aim at, not bank on
Here's the honest part. Very few babies actually arrive on the estimated date. Studies put it at only around 4 or 5 in every 100. Most show up in the two weeks on either side, anywhere from week 37 to 42, and that whole window is normal.
Naegele's rule also assumes a regular 28-day cycle with ovulation around day 14. If your cycles run longer, shorter or irregular, the LMP estimate can drift, which is why an early dating scan often gives a more reliable due date. So treat this number as a guidepost. Your doctor will confirm and may adjust it, and any worry about timing or symptoms is always a conversation worth having with them.
Pregnancy Due Date — frequently asked questions
How is my pregnancy due date calculated?
The common method is Naegele's rule, which adds 280 days, or 40 weeks, to the first day of your last menstrual period. A simpler mental version adds seven days to that date and then counts back three months. Both give the same estimated due date. It assumes a regular 28-day cycle, so an early ultrasound is often used to confirm or refine the date.
Why is pregnancy counted from the last period and not conception?
Because the first day of your last period is a date you can usually remember, while the exact moment of conception is hard to know. Conception typically happens about two weeks after that period starts. Counting from the LMP gives doctors a consistent starting point, which is why a 40-week pregnancy actually involves around 38 weeks of real fetal development.
How accurate is the estimated due date?
It's a good estimate but rarely exact. Only about 4 or 5 babies in every 100 arrive on the predicted date. Most are born within two weeks either side, anywhere from week 37 to 42, all of which is considered normal full term. If your cycles are irregular, an early dating scan tends to give a more accurate due date than the LMP method alone.
What are the three trimesters of pregnancy?
Pregnancy splits into three stages. The first trimester runs through about week 12, when organs form and nausea often peaks. The second covers roughly weeks 13 to 27 and usually feels the most comfortable, with the first movements felt. The third runs from week 28 until birth, when the baby gains weight quickly. Each trimester brings different checkups and changes worth understanding.
What if my menstrual cycle is irregular?
Naegele's rule assumes a steady 28-day cycle, so irregular or longer cycles can throw the LMP estimate off by days or even weeks. In that case the date based on your last period may be unreliable. An early ultrasound, ideally in the first trimester, measures the baby directly and usually gives a more accurate due date. Your doctor will likely use that scan to set the timeline.
Can the due date change during pregnancy?
Yes, and that's normal. If an early scan shows the baby measuring larger or smaller than the LMP suggests, the doctor may adjust the due date to match. The first trimester scan is usually the most reliable for dating. Once set, the date is generally kept, since later scans become less precise for timing. Always go by the date your doctor confirms.
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