This pregnancy calculator estimates your due date and tells you exactly how many weeks pregnant you are today. It uses Naegele's rule — the same method obstetricians have used for two centuries — counting 280 days (40 weeks) from the first day of your last menstrual period, or 266 days from conception if you know that date.
Because not everyone has a textbook 28-day cycle, the calculator also adjusts your due date for cycle lengths from 21 to 35 days. Alongside the due date you get your current gestational age in weeks and days, your trimester, and the dates when each trimester ends.
How Naegele's Rule Works
Naegele's rule, published by German obstetrician Franz Naegele in 1812, sets the estimated due date at:
Due date = first day of last menstrual period (LMP) + 280 days
Pregnancy is dated from the LMP rather than conception because most people know when their period started but not when they ovulated. Since ovulation typically happens about 14 days into a 28-day cycle, the LMP method builds in a two-week head start — you are counted as "2 weeks pregnant" at the moment of conception.
For longer or shorter cycles, ovulation shifts, so the calculator adjusts: due date = LMP + 280 + (cycle length − 28) days. With a known conception date, the formula is simply conception + 266 days, no cycle adjustment needed.
Trimesters and Key Milestones
Pregnancy is divided into three trimesters, each with its own checkpoints:
- First trimester (weeks 1–13): organs form; morning sickness peaks around weeks 8–10; first ultrasound dating scan typically at 8–12 weeks.
- Second trimester (weeks 14–27): the anatomy scan happens around week 20; most people feel first movements between weeks 16 and 22.
- Third trimester (week 28 to birth): glucose screening around weeks 24–28; full term begins at 37 weeks.
A first-trimester ultrasound is more accurate than any date-based calculation, and if the scan differs from the LMP estimate by more than 5–7 days, providers usually re-date the pregnancy to match the scan.
Example: LMP of March 10 with a 30-Day Cycle
Suppose your last period started March 10, 2026, and your cycles average 30 days.
Standard Naegele calculation: March 10 + 280 days = December 15, 2026. Cycle adjustment: 30 − 28 = +2 days, so the adjusted due date is December 17, 2026.
If instead you knew conception occurred March 26, the math would be March 26 + 266 days = December 17, 2026 — the same answer, which makes sense because a 30-day cycle pushes ovulation to about day 16. On July 14, 2026, this pregnancy would measure 17 weeks 5 days (dated from the cycle-adjusted timeline), squarely in the second trimester, with the anatomy scan about two weeks away.
Frequently Asked Questions
How is my due date calculated?
Your due date is 280 days (40 weeks) after the first day of your last menstrual period, per Naegele's rule, adjusted by one day for each day your cycle differs from 28 days. If you know your conception date, the due date is 266 days after conception instead.
How many weeks pregnant am I?
Count the days from the first day of your last period to today and divide by 7 — that is your gestational age in weeks. For example, if your LMP was 10 weeks and 3 days ago, you are 10 weeks 3 days pregnant, even though conception occurred about 2 weeks after that date.
How accurate are due date calculators?
Only about 4% of babies are born on their exact due date, but roughly 90% arrive within two weeks of it. A first-trimester ultrasound is the most accurate dating method, with a margin of about 5 days. Date-based estimates are less reliable if your cycles are irregular.
Why does cycle length change my due date?
Ovulation — and therefore conception — happens about 14 days before your next period, not a fixed 14 days after your last one. With a 35-day cycle you likely ovulated around day 21, a week later than the standard assumption, so your due date shifts about 7 days later.
When does each trimester start and end?
The first trimester runs from week 1 through the end of week 13, the second from week 14 through week 27, and the third from week 28 until birth. Full term is 37 to 42 weeks; babies born before 37 weeks are considered preterm.