Due Date Calculator
Estimate your pregnancy due date and view milestone dates.
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How Due Dates Are Calculated
Naegele's Rule adds 280 days (40 weeks) to the first day of your last menstrual period. Only about 5% of babies are born exactly on their due date — most arrive within 2 weeks before or after. Ultrasound dating, especially before 12 weeks, is more accurate than menstrual dates alone.
Pregnancy Trimesters: What to Expect
First trimester (weeks 1-12): major organ development occurs, morning sickness is common, and miscarriage risk is highest. Second trimester (weeks 13-27): energy often returns, baby movements begin around 18-20 weeks, and the anatomy scan typically occurs at week 20. Third trimester (weeks 28-40): rapid growth, Braxton Hicks contractions may begin, and preparation for birth.
Factors That Affect Your Due Date
First-time mothers tend to go about 4 days past their due date on average. Multiple pregnancies (twins, triplets) typically arrive earlier. If your cycle is shorter or longer than 28 days, the calculator adjusts accordingly. Always follow your healthcare provider's guidance for the most accurate dating.
When to use this
You have just confirmed a pregnancy and the first question everyone asks — including you — is "when is the baby due?" This calculator estimates your due date using the same method your OB-GYN uses: Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). If you know your conception date instead, or if you are using IVF and have a transfer date, the calculator adjusts accordingly.
Knowing your estimated due date is more than satisfying curiosity — it anchors your entire prenatal timeline. It determines when to schedule screenings (the nuchal translucency scan at 11–14 weeks, the anatomy scan at 18–22 weeks), when to start childbirth classes, when to begin maternity leave planning, and when to have your hospital bag packed. This calculator also shows milestone dates for each trimester so you can see the full map at a glance.
It is worth remembering that a due date is an estimate, not an appointment. Only about 4% of babies arrive on their exact due date. Most full-term births occur within a two-week window on either side. Your provider may adjust the date after an early ultrasound, which can measure fetal size and refine the estimate. Consider the date a center point of a range, not a deadline.
Good to know
Naegele's rule assumes a 28-day cycle. The formula adds 280 days from LMP, which assumes ovulation occurred on day 14 of a 28-day cycle. If your cycles are longer or shorter, your actual ovulation — and therefore your due date — may shift by several days. Women with 35-day cycles, for example, likely conceived about a week later than the formula predicts.
First-trimester ultrasound is the most accurate dating method. Crown-rump length measured between 8 and 13 weeks can estimate gestational age within about 5 days. If the ultrasound date differs from your LMP-based date by more than 7 days, most providers will use the ultrasound date instead.
IVF dates are the most precise. Because the exact day of embryo transfer is known, IVF due dates have a smaller margin of error than LMP-based estimates. For a day-5 blastocyst transfer, the due date is typically calculated as the transfer date plus 261 days.
Full term is a range, not a single week. The American College of Obstetricians and Gynecologists defines "early term" as 37–38 weeks, "full term" as 39–40 weeks, "late term" as 41 weeks, and "post-term" as 42 weeks and beyond. Deliveries within the full-term window are generally associated with the best outcomes.
This calculator is informational, not medical. It provides the same basic calculation your provider uses, but it cannot account for individual medical factors. Always confirm your due date and prenatal care plan with your healthcare team.
Quick Reference
| Milestone | Timing | Notes |
|---|---|---|
| First Trimester | Weeks 1 – 12 | Organ formation, early screenings |
| Second Trimester | Weeks 13 – 26 | Anatomy scan, gender reveal possible |
| Third Trimester | Weeks 27 – 40 | Growth, positioning, delivery prep |
| Viability milestone | ~24 weeks | Survival outside womb becomes possible |
| Early Term | 37 – 38 weeks | Lungs typically mature by 36–37 weeks |
| Full Term | 39 – 40 weeks | Optimal delivery window |
| Post-Term | 42+ weeks | Induction typically discussed |