Pregnancy Due Date Calculator

How Is the Pregnancy Due Date Calculated?

Your pregnancy due date is formally called the Estimated Due Date (EDD) or Estimated Date of Delivery (EDD). The most common calculation method is Naegele's Rule — take the first day of your last menstrual period (LMP), add 280 days (exactly 40 weeks). This assumes a regular 28-day cycle with ovulation occurring on day 14. Our free pregnancy due date calculator supports three methods: LMP, known conception date, and IVF transfer date (for both Day 3 and Day 5 blastocyst transfers).

Only about 5% of babies are born exactly on their estimated due date — approximately 80% of births occur between weeks 37 and 42. The tool also tracks your current gestational age in weeks and days, your trimester, pregnancy progress percentage, days remaining, and five key milestone dates.

How to Calculate Your Pregnancy Due Date Online

1. Choose your calculation method — select Last Period, Conception, or IVF using the three-button toggle.
2. Last Period method — enter the first day of your most recent menstrual period. The calculator uses Naegele's Rule (LMP + 280 days).
3. Conception method — enter your estimated conception date. The calculator adds 266 days (38 weeks from conception = 40 weeks from LMP).
4. IVF method — enter your embryo transfer date, then select Day 3 or Day 5 embryo age. Day 5 blastocysts: transfer date + 261 days. Day 3 embryos: transfer date + 263 days.
5. Read your results — your estimated due date, gestational age in weeks + days, trimester card, days remaining, and all five milestone dates appear instantly.

How the Due Date Calculator Works Technically

The calculator uses JavaScript's Date object running in the browser via React useMemo. Date manipulation uses the addDays helper which creates a new Date and sets .getDate() + n — correctly handling month-end rollovers and leap year February. Gestational age is computed by finding the days elapsed from the conceptional start of the 280-day window to today, dividing by 7 for weeks and taking the remainder for extra days. Trimester assignment uses week thresholds: <13 weeks = first, 13–27 = second, 28+ = third.

Understanding Pregnancy Trimesters — 6 Key Facts

• First Trimester (Weeks 1–12): Major organ formation occurs. This is when morning sickness, fatigue, and breast tenderness are most common. The miscarriage risk is highest during this period and drops significantly after week 12 once the placenta fully takes over.
• First trimester screening: The dating and nuchal translucency scan is typically offered between weeks 11 and 14. Combined with blood tests, this screens for chromosomal conditions including Down syndrome.
• Second Trimester (Weeks 13–27): Often called the golden period — nausea typically subsides, energy returns, and foetal movements become noticeable (quickening) between weeks 16 and 24 for most women.
• Anatomy scan at Week 20: The mid-pregnancy structural scan checks foetal anatomy in detail — brain, heart, spine, limbs, and organs. The sex of the baby can often be seen at this appointment if desired.
• Third Trimester (Weeks 28–40): The baby gains most of its birth weight during this period. Braxton Hicks practice contractions may begin. Hospital bags should be packed by week 36 as early labour becomes possible.
• Full term at Week 37: Babies born from week 37 onwards are considered medically full term. Early term (37–38 weeks) babies may face slightly higher risks than those born at 39–40 weeks. Use our Age Calculator to track milestone anniversaries after delivery.

Why 40 Weeks? The History of Naegele's Rule

The 40-week standard was established by German obstetrician Franz Karl Naegele in the early 19th century based on observations of typical pregnancy length. The two "extra" weeks beyond conception are counted because the exact conception date is almost always unknown, while the LMP is more reliably recalled. The 280-day total (LMP to birth) encompasses the approximately 14 days from LMP to ovulation/conception, plus the 266 days of actual embryonic and foetal development.

Ultrasound Dating vs LMP Dating

An early ultrasound at 7–12 weeks is the most accurate dating method available. The crown-rump length (CRL) measurement of the embryo correlates precisely with gestational age in early pregnancy. If ultrasound dating differs from LMP-based dating by more than 5 days in the first trimester, the ultrasound date is typically adopted as the official EDD. This is why your doctor's confirmed due date may differ slightly from what this calculator shows — and their date should always take precedence. Pair with our Water Intake Calculator to ensure optimal hydration during pregnancy.

Privacy and Security

The Pregnancy Due Date Calculator runs entirely in your browser using JavaScript's built-in Date API — no library, no API call, no server request. Your LMP date, conception date, IVF transfer date, and all pregnancy timeline calculations never leave your device. This is safe for early pregnancy planning, sensitive reproductive health information, IVF cycle tracking, and personal pregnancy data you prefer not to submit to external online services.

✓Verified by ToollyX Team · Last updated June 2026