Enter the first day of your last period to estimate your due date
The standard method for estimating a due date is Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation occurring on day 14. If your cycle is longer or shorter than 28 days, the calculator adjusts the estimate accordingly — a 35-day cycle shifts the due date forward by 7 days, since ovulation likely happened later.
The 280-day figure is a population average. Only about 4% of babies are born on their exact due date. Most full-term deliveries happen within two weeks of the estimate — between 38 and 42 weeks of gestation. Think of your due date as the center of a window, not a deadline.
| Trimester | Weeks | Key developments |
|---|---|---|
| First (weeks 1–12) | Weeks 1–12 | Implantation, heartbeat detectable (week 6), major organs form, nausea peaks |
| Second (weeks 13–26) | Weeks 13–26 | Gender visible on ultrasound (week 18–20), quickening (first movements felt), anatomy scan |
| Third (weeks 27–40) | Weeks 27–40 | Rapid weight gain, lungs mature, baby moves into head-down position, nesting instinct |
| Week | Milestone |
|---|---|
| Week 6 | Heartbeat detectable on ultrasound |
| Week 12 | End of first trimester — miscarriage risk drops significantly |
| Week 20 | Anatomy scan — halfway point |
| Week 24 | Viability threshold — survival outside the womb becomes possible with intensive care |
| Week 28 | Third trimester begins — kick counting recommended |
| Week 37 | Early term — baby is considered mature enough for delivery |
| Week 39–40 | Full term — ideal delivery window |
| Week 42 | Post-term — induction typically recommended |
The default 28-day cycle assumption does not apply to everyone. Cycles between 21 and 35 days are considered normal. If your cycle is consistently 32 days, ovulation likely occurs around day 18 rather than day 14 — pushing your actual conception date (and therefore your due date) later than Naegele's Rule would suggest. Enter your typical cycle length in the calculator above for a more accurate estimate.
Irregular cycles make due date estimation less reliable from LMP alone. In these cases, a dating ultrasound in the first trimester (typically at 8–12 weeks) provides the most accurate estimate by measuring the embryo's crown-rump length.
Pregnancy is measured from the first day of the last menstrual period, not from conception. This means you are technically considered 2 weeks pregnant at the time of conception. The system exists because LMP is a date most people can identify with certainty, while the exact day of conception is usually unknown.
If you do know your conception date, subtract 14 days to approximate your LMP, or simply count 266 days (38 weeks) forward from conception to estimate your due date.
Doctors may adjust the due date based on first-trimester ultrasound if the measurement differs from the LMP calculation by more than 7 days. After the first trimester, ultrasound dating becomes less accurate because fetal growth rates vary more between individuals. The ACOG (American College of Obstetricians and Gynecologists) recommends using the earliest reliable dating method as the final estimate.
Due date calculators provide an estimate accurate to within about 2 weeks for most pregnancies. Only 4% of babies arrive on the exact due date. The calculator is most accurate when your cycle length is regular and known. A first-trimester ultrasound can refine the estimate further.
Yes. If a dating ultrasound in the first trimester shows a gestational age that differs from the LMP-based calculation by more than 7 days, most providers will adjust the due date to match the ultrasound measurement.
If the date of your last menstrual period is unknown, a dating ultrasound performed between 8 and 12 weeks of pregnancy provides the most accurate alternative for estimating due date and gestational age.
Full term is defined as 39 weeks 0 days through 40 weeks 6 days. Early term is 37–38 weeks, late term is 41 weeks, and post-term is 42 weeks and beyond. Delivery at full term is associated with the best outcomes for both mother and baby.
For IVF, the due date is calculated from the embryo transfer date rather than LMP. A Day 5 blastocyst transfer adds 261 days (38 weeks minus 5 days) to estimate the due date. This method is more precise than LMP-based dating because the exact date of conception is known.