One of the very first questions after a positive pregnancy test is simple: when is the baby coming? Doctors and midwives have used the same basic method for nearly two centuries, and it's easy to understand once you see how it works.
Naegele's rule: 280 days
The standard way to estimate a due date is Naegele's rule:
Due date = first day of your last period + 280 days (40 weeks)
For example, if your last period started on January 1, your estimated due date is October 8.
It can feel strange that the count starts before you were actually pregnant. The reason is practical: most people don't know the exact day they conceived, but they usually remember when their last period began. Conception typically happens about two weeks after that, so "40 weeks pregnant" really means about 38 weeks since conception.
Adjusting for your cycle length
The 280-day rule assumes a 28-day cycle, with ovulation around day 14. If your cycles are longer or shorter, ovulation shifts too — so the due date shifts by the same amount:
- 32-day cycle: add 4 days to the standard due date
- 25-day cycle: subtract 3 days
If you know your conception date
If you know when conception happened (for example, after IVF or ovulation tracking), count forward 266 days (38 weeks) instead.
Weeks, trimesters, and what "how far along" means
Pregnancy is usually tracked in weeks and days, like "12w 3d." It's also split into three trimesters:
- 1st trimester: weeks 0–12
- 2nd trimester: weeks 13–26
- 3rd trimester: week 27 until birth
How to use it
The Due Date Calculator handles all of this for you. Choose "Last period" or "Conception date," enter the date, and set your usual cycle length if it isn't 28 days. You'll see your estimated due date, how many weeks and days along you are today, your current trimester, and how many days are left to go.
A few things worth knowing:
- Due dates are estimates. Only about 4–5% of babies arrive exactly on the day. Anywhere from 37 to 42 weeks is common.
- Early ultrasounds may adjust it. A dating scan in the first trimester is often more accurate than a period-based estimate.
- Plan in ranges. Think "early October," not "October 8th."
For planning around the big day, the Date Difference Calculator counts the days between any two dates, and the Countdown Timer keeps a live countdown you can check anytime.
This tool is for general information only and isn't medical advice. Your doctor or midwife will confirm your due date and guide your prenatal care.