Pregnancy Due Date Calculator

Calculate your expected delivery date, current trimester, and week-by-week pregnancy timeline from your last menstrual period

Your Results

Enter your last menstrual period date and click Calculate.

Pregnancy Timeline
TrimesterWeeksKey Milestones
1st TrimesterWeeks 1–12Implantation, heartbeat detected (~week 6), major organs form
2nd TrimesterWeeks 13–26Baby moves (~week 18–20), gender visible on ultrasound, rapid growth
3rd TrimesterWeeks 27–40Baby gains weight, lungs mature, head engages, birth preparation

Frequently Asked Questions

The most common method 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. If your cycle is longer or shorter, the calculator adjusts by adding or subtracting the difference from 28 days.

Only about 5% of babies are born exactly on their due date. Most births occur within 2 weeks before or after the estimated due date. A full-term pregnancy is considered anywhere from 39 to 40 weeks and 6 days. An early ultrasound (between 8–13 weeks) provides the most accurate dating.

LMP stands for Last Menstrual Period. Since the exact date of conception is rarely known, doctors count from the first day of your last period. Ovulation typically occurs about 14 days after LMP, so actual conception is roughly 2 weeks after the LMP date.

The first trimester runs from weeks 1–12, the second from weeks 13–26, and the third from weeks 27–40. The first trimester is the most critical for fetal development, when major organs and systems are forming.

If you don't know your LMP date, you can use the conception date option in this calculator if you know when conception occurred. Alternatively, a doctor can estimate gestational age through an ultrasound, which measures the size of the embryo or fetus.

Naegele's Rule is the standard medical formula for estimating a pregnancy due date. Formulated by German obstetrician Franz Karl Naegele in 1812, it calculates the estimated due date (EDD) as: first day of last menstrual period (LMP) + 280 days (40 weeks). In practice this equals: LMP + 1 year − 3 months + 7 days. The formula assumes a 28-day cycle and that ovulation occurs on day 14. For cycles longer or shorter than 28 days, the due date is adjusted accordingly. Despite its age, Naegele's Rule remains the primary dating method used by clinicians worldwide, often confirmed and refined by early ultrasound. Always consult your healthcare provider for your personalised due date.

Pregnancy is counted from the first day of the last menstrual period (LMP) rather than from conception because the exact date of conception is rarely known with certainty — it depends on ovulation timing and the lifespan of sperm, neither of which are directly observable. The LMP date, by contrast, is something most people can recall or record. In a standard 28-day cycle, ovulation occurs around day 14, so actual conception typically happens approximately 2 weeks after LMP. This means the first two weeks of a "40-week pregnancy" are technically before conception occurred. The convention ensures a consistent, reproducible reference point for gestational age that all clinicians can use.

Due date predictions are an estimate, not a guarantee. Only about 5% of babies are born on their exact due date. Most babies are born within 2 weeks before or after the estimated due date — the normal range for a full-term birth is 37 to 42 weeks of gestation. Accuracy depends on the dating method: LMP-based dating alone has a margin of ±2 weeks. A first-trimester ultrasound (before 14 weeks) is more accurate, with a margin of approximately ±5–7 days. Second and third trimester ultrasounds are less accurate for dating. Consult your healthcare provider about your individual dating accuracy and expected delivery window.

Earlier ultrasounds are more accurate for establishing gestational age. In the first trimester (6–13 weeks), fetal size is largely determined by genetics and gestational age, not yet by maternal nutrition or fetal growth patterns. Crown-rump length (CRL) measurement at this stage provides a dating accuracy of approximately ±5–7 days. By the second trimester (14–28 weeks), individual growth variation increases, reducing accuracy to approximately ±10–14 days. In the third trimester, accuracy is only ±2–3 weeks. For this reason, if an early ultrasound and LMP-based dating differ by more than 7 days, clinicians typically revise the due date to match the ultrasound. Always discuss dating results with your healthcare provider.

A pregnancy that continues beyond 42 weeks of gestation (2 weeks past the estimated due date) is classified as post-term. A pregnancy between 41 weeks 0 days and 41 weeks 6 days is called late-term. Post-term pregnancies carry increased risks including placental insufficiency (the placenta ages and becomes less effective at supplying nutrients and oxygen), meconium aspiration, and increased risk of stillbirth. For this reason, most obstetric guidelines recommend increased monitoring from 41 weeks (non-stress tests, biophysical profiles, amniotic fluid assessment) and consideration of labour induction between 41 and 42 weeks. Management decisions should be made in consultation with your healthcare provider based on individual circumstances.

About This Pregnancy Due Date Calculator

This free Pregnancy Due Date Calculator uses Naegele's Rule — the same method used by doctors worldwide. Enter the first day of your last menstrual period to get your estimated due date, current weeks pregnant, trimester, and key milestone dates. Supports variable cycle lengths between 21 and 35 days.

Weeks Pregnant Reference

WeeksTrimesterApprox. Size
4 weeks1stPoppy seed
8 weeks1stRaspberry
12 weeks1stLime
16 weeks2ndAvocado
20 weeks2ndBanana
24 weeks2ndCorn
32 weeks3rdSquash
40 weeks3rdWatermelon

Key Milestone Dates

MilestoneTypical Week
Heartbeat detectableWeek 6
End of 1st trimesterWeek 12
Anatomy scan (ultrasound)Week 18–20
Viability milestoneWeek 24
End of 2nd trimesterWeek 26
Early term birthWeek 37
Full term birthWeek 39–40

Standards & References

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