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Due Date Calculator

Calculate your estimated due date. Trimester, week, and key milestones included.

Calculation method

Naegele's rule: Due date = LMP + 280 days

This tool is for informational purposes only and is not a substitute for professional medical advice. Consult your doctor or midwife for personalized care.

How the Due Date Calculator Works

This calculator uses Naegele's rule, the standard method used by most healthcare providers. Enter the first day of your last menstrual period (LMP) and the calculator adds 280 days (40 weeks) to estimate your due date.

The tool also shows your current week of pregnancy, your trimester, how many days remain until your due date, and key milestones like the first ultrasound and anatomy scan. Worth noting: only about 5% of babies are born on their exact due date, the estimate is a 40-week midpoint, not a deadline.

Pregnancy Trimesters at a Glance

  • 1st trimester (weeks 1–12): The embryo develops all major organs. Fatigue and nausea are common. The risk of miscarriage is highest in this period, which is why many wait until the end of the first trimester to share the news.
  • 2nd trimester (weeks 13–27): Often called the "honeymoon trimester" — nausea typically subsides, energy returns, and the baby bump becomes visible. The anatomy scan around week 20 checks fetal development in detail.
  • 3rd trimester (weeks 28–40): Rapid growth and weight gain for the baby. Sleep becomes harder, and Braxton Hicks contractions may begin. At 37 weeks the baby is considered full term. Most births happen between weeks 38 and 42.

Four methods, six milestones, one date

Most due date tools only do LMP. This one handles four calculation methods with the correct day offsets for each: Last Menstrual Period (+280 days, Naegele's rule), Known Conception date (+266 days), IVF Day-3 transfer (+263 days), and IVF Day-5 transfer (+261 days). The IVF offsets matter — a Day-5 blastocyst transfer is already 5 days developed, so the arithmetic is different. Enter your transfer date and the right number is used automatically.

The results include your current trimester, current week of pregnancy, days remaining until the due date, and six milestones with check marks as you pass them: 6 weeks (heartbeat first detectable), 12 weeks (end of first trimester), 20 weeks (anatomy scan), 24 weeks (viability threshold), 37 weeks (full term), and 40 weeks (due date). What's not here: contraction timing, kick counters, weight gain charts, or anything requiring an account.

How Pregnancy Due Date is Calculated

Naegele's rule, the standard since the 1800s, is simple: take the first day of your last menstrual period (LMP) and add 280 days (40 weeks). That gives your estimated due date (EDD). The rule assumes a 28-day cycle with ovulation on day 14. If your cycle is consistently longer or shorter, the EDD shifts accordingly — a 35-day cycle typically pushes the due date 7 days later.

Trimester breakdown: First trimester covers weeks 1–12 (major organ formation, highest miscarriage risk). Second trimester spans weeks 13–26 (energy returns, anatomy scan at ~week 20). Third trimester runs weeks 27–40+ (rapid fetal growth, preparation for birth). Only 4–5% of babies are born on their exact due date; about 80% arrive somewhere between weeks 38 and 42.

First-trimester ultrasound (measuring crown-rump length at 11–14 weeks) is generally more accurate than Naegele's rule, with an error margin of ±5–7 days. It is especially useful for people with irregular cycles or uncertain LMP dates. When the ultrasound date differs from the LMP-based estimate by more than 7 days, providers typically adopt the ultrasound date.

Related tools: Age Calculator, Date Calculator, and Sleep Calculator.

Limitations of Due Date Estimation

The EDD (estimated due date) is a statistical median, not a target. Medical terminology distinguishes: early term (37–38 weeks), full term (39–40 weeks), late term (41 weeks), and post-term (42+ weeks). Babies born at 39–40 weeks typically have the best outcomes, the due date marks the midpoint of a natural range, not a deadline.

Irregular menstrual cycles reduce the accuracy of LMP-based dating. For assisted reproduction (IVF), the embryo transfer date is used instead of LMP — this calculator supports IVF Day-3 and Day-5 transfer offsets for that reason. A missed due date doesn't mean something is wrong; gestational age measurement has an inherent ±1–2 week uncertainty even with ultrasound.

This calculator uses LMP-based Naegele's rule as its default method — consult your healthcare provider for individualized dating. Canadian SOGC guidelines recommend routine first-trimester ultrasound at 11–14 weeks to confirm gestational age for all pregnancies.

Frequently Asked Questions

How accurate is the due date calculation?
Naegele's rule gives an estimate based on a 28-day cycle. It assumes ovulation occurs on day 14, which isn't true for everyone. Ultrasound dating, especially in the first trimester, is generally more accurate because it measures actual fetal size. Only about 5% of babies are born on their exact due date — most are born within two weeks before or after.
What if I don't know my last menstrual period date?
If you don't know your LMP date, your healthcare provider will typically use an early ultrasound to estimate gestational age by measuring the fetus. The crown-rump length measured in the first trimester is the most reliable method when LMP is unknown or uncertain.
Is a baby born at 37 weeks considered full term?
Yes — 37 weeks is the threshold for full term. However, medical guidelines distinguish between early term (37–38 weeks), full term (39–40 weeks), late term (41 weeks), and post term (42+ weeks). Babies born at 39–40 weeks generally have the best outcomes. If your due date is approaching or has passed, consult your healthcare provider.
What if my cycle isn't 28 days?
Naegele's rule assumes a 28-day cycle and ovulation on day 14. If your average cycle is 35 days, you likely ovulate on day 21, so your due date is 7 days later than the standard calculation. Many calculators (and this one) let you enter your average cycle length to adjust automatically.
How accurate is an ultrasound due date vs the last period?
A first-trimester ultrasound (measuring crown-rump length at 11–14 weeks) is more accurate than LMP-based dating, with an error margin of ±5–7 days. By the third trimester, ultrasound accuracy drops to ±3–4 weeks. If your ultrasound date differs from your LMP date by more than 7 days, your doctor will typically use the ultrasound date.

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By Bam's Thinkery — Updated

Informational tool. Not a substitute for advice from a qualified healthcare professional.