Pregnancy Due Date Calculator
Enter the first day of your last menstrual period (LMP) and your cycle length to estimate your due date using Naegele's rule. You will also see how many weeks and days pregnant you are today, the D-day countdown to delivery, and which trimester you are currently in.
This result is for reference only and is not a medical diagnosis. Confirm your exact due date with your obstetrician.
Examples
LMP on January 1, 2026 with a 28-day cycle
The due date is 280 days later: October 8, 2026. With a 28-day cycle, no adjustment is needed — simply add 280 days to the LMP.
LMP on January 1, 2026 with a 35-day cycle
Since the cycle is 7 days longer than 28, ovulation is assumed to be 7 days later, so 7 extra days are added. The due date is October 15, 2026.
Today is day 100 after the LMP (28-day cycle)
That corresponds to 14 weeks 2 days of pregnancy — the very beginning of the second trimester (weeks 14–27). There are 180 days left until the due date.
FAQ
What is Naegele's rule?
It is a due-date formula proposed by the German obstetrician Franz Naegele: add 280 days (40 weeks) to the first day of the last menstrual period. If the menstrual cycle is not 28 days, the result is adjusted by adding (cycle length − 28) days. It is the standard baseline method used by obstetricians worldwide.
What if my menstrual cycle is irregular?
Naegele's rule assumes a regular cycle, so an irregular cycle can make the result considerably less accurate. In that case, an early ultrasound (weeks 8–13) that measures the fetus's crown-rump length (CRL) gives a much more reliable due date. Enter your average cycle length and treat the result as a rough reference only.
Why is my ultrasound due date different from this calculation?
Naegele's rule is a statistical estimate based on the last menstrual period, while an ultrasound due date is based on the actual measured size of the fetus. Differences arise when ovulation occurred earlier or later than assumed, or when the LMP date is not remembered exactly. When the two disagree, the early-ultrasound date is generally the one used.
How many babies are actually born on their due date?
Only about 5% of babies are born on the exact due date. Most arrive within two weeks before or after it (weeks 38–42 of pregnancy), and births in this window are all considered normal. The due date is a reference point for managing the pregnancy, not a prediction of the exact birth day.
How are the weeks of pregnancy divided?
Pregnancy is counted from the first day of the last menstrual period and divided into three trimesters: the first trimester runs through week 13, the second covers weeks 14–27, and the third from week 28 until delivery. Birth before 37 weeks is preterm, 37–41 weeks is full term, and 42 weeks or later is post-term.
How the due date is calculated
The due date is found by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). Conception actually happens around ovulation (roughly 14 days before the next expected period), but because the ovulation date is hard to pin down, the easy-to-remember LMP is used as the standard starting point.
If the cycle differs from 28 days, ovulation shifts by the same amount, so (cycle length − 28) days are added as a correction. For example, add 2 days for a 30-day cycle or subtract 2 days for a 26-day cycle. The same correction is applied when calculating the current week of pregnancy.
Trimester overview
In the first trimester (through week 13) the baby's major organs form; morning sickness and fatigue are common, and folic acid intake and early screening tests matter most. The second trimester (weeks 14–27) is a relatively stable period when nausea eases and fetal movements begin, with the detailed anatomy scan and gestational diabetes test taking place.
In the third trimester (week 28 onward) the baby grows rapidly and prepares for birth. Fetal movement is monitored, prenatal checkups become more frequent, and from week 37 labor can start at any time, so it is wise to have everything ready for delivery.
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