Pregnancy Due Date Calculator
Calculates your estimated due date, current gestational age, and key pregnancy milestones from the first day of your last menstrual period and your cycle length. Use it as soon as you suspect pregnancy or receive a positive test.
About this calculator
Naegele's Rule is the standard method for estimating a due date: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a standard 28-day cycle. When a cycle differs from 28 days, the formula adjusts: Due Date = LMP + (280 + cycleLength − 28) days. The adjustment shifts ovulation—and therefore conception—earlier or later relative to the LMP. For example, a 35-day cycle means ovulation occurs about 7 days later, so 7 extra days are added. Gestational age is counted from the LMP, not from conception, which is why pregnancy is described as 40 weeks even though fertilization typically occurs around week 2. Ultrasound dating in the first trimester remains the most accurate method, but LMP-based calculation is an excellent starting estimate used worldwide.
How to use
Suppose your LMP started on March 15, 2025, and your average cycle length is 32 days. Step 1: Calculate the adjusted gestational period: 280 + (32 − 28) = 284 days. Step 2: Add 284 days to March 15, 2025. March has 16 remaining days after the 15th; 284 − 16 = 268 days into April onward. Counting forward, 284 days from March 15 lands on January 23, 2026. Step 3: Your estimated due date is January 23, 2026. Gestational age on any given day is the number of days since March 15 divided by 7 to get weeks.
Frequently asked questions
How accurate is a due date calculated from the last menstrual period?
LMP-based due dates are accurate to within about 1–2 weeks for most women with regular cycles. Only about 5% of babies are born exactly on their estimated due date; the majority arrive within two weeks before or after. Accuracy decreases for women with irregular cycles, unknown LMP dates, or recent hormonal contraceptive use. First-trimester ultrasound (between 8 and 13 weeks) is considered the gold standard for dating accuracy, with a margin of error of ±5–7 days. When LMP and ultrasound dates differ by more than 7 days, clinicians typically defer to the ultrasound.
Why does cycle length affect the pregnancy due date calculation?
Naegele's Rule assumes ovulation occurs on day 14 of a 28-day cycle, which is when fertilization most likely happens. If your cycle is longer—say 35 days—ovulation occurs around day 21, meaning conception happens about 7 days later than assumed. Adding those extra days to the standard 280 corrects for this delay. Conversely, a shorter cycle (e.g., 21 days) means earlier ovulation, so the due date moves earlier. This cycle-length adjustment is a simple but effective way to personalise the estimate without requiring hormone tracking or ultrasound.
What are the important pregnancy milestones I should track after calculating my due date?
Key milestones include week 6–8 (first heartbeat detectable by ultrasound), week 10–13 (first-trimester screening and nuchal translucency scan), week 20 (anatomy scan to check fetal development and confirm sex if desired), and week 28 (start of third trimester and glucose tolerance testing). Week 37 marks full term, meaning the baby is considered ready for birth without special intervention. Knowing your due date allows you to schedule these appointments on time, which is critical for catching potential complications such as chromosomal abnormalities, gestational diabetes, and preeclampsia early.