How many weeks pregnant are you? Find out instantly with New Mom Choice’s Pregnancy week Calculator. Enter your last period, conception, or due date to see your exact week, current trimester, and countdown to baby’s arrival. Because is simple, accurate, and free — the easiest way for new moms to track their pregnancy journey.
Pregnancy Week Calculator
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For informational purposes only — not medical advice. See our Medical & Health Disclaimer.
How Your Due Date Is Calculated
Wondering how our pregnancy week calculator comes up with your due date? Here’s the simple math behind it — because there is no guesswork, just the same method your doctor uses.
The Standard Method: Naegele’s Rule
Using this pregnancy week calculator your due date is estimated using the standard method doctors have relied on for decades: counting 280 days (40 weeks) from the first day of your last menstrual period (LMP). This is called Naegele’s Rule, and it assumes a typical 28-day cycle with ovulation happening around day 14.
This isn’t just an internet formula — ACOG (the American College of Obstetricians and Gynecologists) confirms that by convention, the estimated due date is 280 days after the first day of the LMP, and this is the starting point used at your very first prenatal appointment. It’s not perfect — no formula can predict the exact day your baby decides to arrive — and ACOG itself notes this method however it doesn’t account for irregular cycles or inaccurate recall of the LMP. In fact, research cited by ACOG suggests only about half of women can accurately recall their LMP, which is one reason your doctor will usually double-check dating with an early ultrasound.
Three Ways to calculate Your Estimate
Not everyone tracks the first day of their last period, so the pregnancy week calculator gives you flexibility:
- Enter your LMP → the calculator counts forward 280 days to your due date.
- Enter your conception date → the calculator simply adds 14 days backward to estimate your LMP, then calculates forward the same way.
- Enter your due date directly (say, from an ultrasound) → it works backward to figure out your current week.
However you enter it, you’ll land on the same week-by-week timeline.
Why Your Doctor May Adjust the Date
If you’ve had (or will have) a first-trimester ultrasound, don’t be surprised if your due date shifts slightly. ACOG considers first-trimester ultrasound the most accurate way to confirm gestational age — more reliable than LMP-based dating alone. That’s simply because early fetal measurements are very consistent from pregnancy to pregnancy, while cycle length and ovulation timing vary a lot from woman to woman.
A Quick Reminder
This pregnancy week calculator gives an estimate, not a guarantee. Because research shows only about 4–5% of babies arrive on their exact calculated due date — most show up within two weeks before or after. Your doctor’s estimate, especially after an early ultrasound, however it is always the most accurate one.
Are you ready for the next step check the Newborn Feeding Guide
Sources:
- ACOG Committee Opinion No. 700: Methods for Estimating the Due Date
- Natural Womanhood: Pregnancy Due Date Accuracy
- Momly: Understanding Naegele’s Rule
Written by Fran & Fabio, founders of New Mom Choice. Fran is a dentist. Fabio is a retired agricultural pilot. Together, they research and fact-check every guide on this site against trusted medical sources — including the AAP and WHO — before it goes live.