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ZeroFee Tools' free pregnancy due date calculator estimates your baby's due date from the first day of your last menstrual period using Naegele's rule, the standard clinical method. You also get your current gestational week and day, trimester, and estimated conception date.
Enter your last menstrual period
Estimated due date
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You are
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Trimester
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Conception (est.)
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Pregnancy milestones
Note: Estimates for information only and not medical advice. Dating is most accurate with an early ultrasound; always confirm with your doctor or midwife and follow their care plan.
Frequently asked questions
What is Naegele's rule?
Naegele's rule estimates the due date as the last menstrual period plus 280 days (40 weeks). It is the standard clinical starting point, assuming a 28-day cycle with ovulation around day 14.
Why is pregnancy counted from the last period instead of conception?
The last period is an observable date, while conception usually is not. Counting from it standardizes care, even though the first two "weeks" precede ovulation.
What are the three trimesters?
First trimester: weeks 1-13. Second: weeks 14-27. Third: weeks 28 to birth. Each brings different checkups, screenings and milestones.
My cycles are irregular. Is the result still useful?
Less so. The rule assumes ovulation on day 14 of a 28-day cycle. With irregular cycles, an early ultrasound dates the pregnancy more accurately.
What is the estimated conception date shown?
Roughly 14 days after the last period (266 days before the due date) for a textbook 28-day cycle. Actual conception varies with your real ovulation day.
How is a pregnancy due date calculated?
Short answer: Clinicians use Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. Equivalently, subtract 3 months, add 7 days and add a year. It assumes a 28-day cycle with ovulation on day 14.
Pregnancy months by week
Month
Weeks
Trimester
Month 1
Weeks 1 - 4
First
Month 2
Weeks 5 - 8
First
Month 3
Weeks 9 - 13
First
Month 4
Weeks 14 - 17
Second
Month 5
Weeks 18 - 22
Second
Month 6
Weeks 23 - 27
Second
Month 7
Weeks 28 - 31
Third
Month 8
Weeks 32 - 35
Third
Month 9
Weeks 36 - 40
Third
Dating starts at the last period, before conception even happens, which is why the "first two weeks" of pregnancy are pre-conception. An early ultrasound (weeks 8-12) dates pregnancy more precisely than the rule when cycles are irregular.
What week of pregnancy am I in right now?
Short answer: Count full weeks since the first day of your last period: that is your gestational week, and the leftover days are the "+ days". For example, 90 days after the last period is 12 weeks and 6 days, written 12w6d.
Enter your last period above and the calculator shows your exact week and day today, plus which trimester you are in. Trimesters run weeks 1-13, 14-27, and 28 through birth.
How accurate are due date calculators?
Short answer: They are good estimates, not appointments: only about 5% of babies are born on their due date, and most arrive within two weeks either side. Accuracy depends on knowing the true first day of the last period and having roughly 28-day cycles.
With irregular cycles, recent hormonal birth control, or uncertain dates, an early ultrasound gives the most reliable dating. Your provider may adjust the due date after the first scan.
How to use this calculator step by step
Enter the first day of your last menstrual period.
Click "Calculate due date" to see your estimated due date.
Read your current gestational week, trimester and estimated conception date.
Review the milestone list for what each trimester brings.
Confirm everything with your doctor or midwife, ideally with an early ultrasound.
