Your estimated due date (EDD) is calculated using Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). If your last period started March 15, your due date is approximately December 20. Only 5% of babies are born on their exact due date — most arrive within 2 weeks before or after. The due date shapes prenatal scheduling, maternity leave planning, and nursery preparation throughout the pregnancy.
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What Is a Pregnancy Due Date Calculator?
A pregnancy due date calculator is a tool that estimates when your baby will be born based on the first day of your last menstrual period (LMP) and your average cycle length. It uses Naegele's Rule — the standard obstetric method for estimating the expected date of delivery (EDD) — to add 280 days (40 weeks) to your LMP date.
The calculation accounts for the fact that pregnancy is measured from the LMP, not from the actual date of conception. Because ovulation typically occurs around Day 14 of a 28-day cycle, the LMP date is approximately two weeks before fertilization. This is why pregnancy is counted as 40 weeks total, even though fetal development spans only about 38 weeks from conception.
For women with cycles longer or shorter than 28 days, the calculator adjusts the due date accordingly. A 35-day cycle adds 7 days to the standard EDD; a 21-day cycle subtracts 7 days. This adjustment matters because cycle length reflects when ovulation — and therefore conception — is most likely to have occurred.
Beyond the due date, the calculator displays current gestational age in weeks and days, the estimated start of each trimester, the anatomy scan window (weeks 18–20), the viability threshold (week 24), and the full-term range (weeks 37–42). These milestones help you prepare for appointments, understand your pregnancy timeline, and know when delivery would be considered premature, early, full term, or post term.
How to Use the Pregnancy Calculator
- Enter the first day of your last menstrual period (LMP). This is the standard reference date for pregnancy dating. If you do not remember the exact date, use the approximate date.
- Enter your average cycle length (optional). Naegele's Rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer (e.g., 35 days), the calculator adjusts the due date forward. If shorter (e.g., 24 days), it adjusts backward.
- Or enter conception date / IVF transfer date. If you know the exact conception date (from ovulation tracking or IVF), the calculator adds 266 days (38 weeks from conception).
- Read the results. Estimated due date, current gestational age in weeks and days, and dates for key milestones: first trimester end (week 13), anatomy scan (week 18–20), third trimester start (week 28), and full-term range (weeks 37–42).
The Due Date Formula
Naegele's Rule:
EDD = LMP + 280 days (40 weeks)
Or equivalently:
EDD = LMP + 1 year + 7 days − 3 months
Worked example:
- LMP: January 10
- Add 280 days: October 17
- Or: January 10 + 1 year = January 10 next year − 3 months = October 10 + 7 days = October 17 ✓
Adjusting for cycle length: If your average cycle is not 28 days, adjust the due date:
Adjustment (days) = Cycle Length − 28
EDD = Standard EDD + Adjustment
If your cycle is 35 days: add 7 days to the standard EDD. If your cycle is 21 days: subtract 7 days from the standard EDD.
Pregnancy Trimester Schedule
| Period | Weeks | Key Events | |--------|-------|-----------| | First Trimester | 1–12 | Fertilization, implantation, embryo → fetus transition, organ formation | | First Trimester End | Week 13 | Miscarriage risk drops significantly (to ~1–2%) | | Second Trimester | 13–26 | Growth, movement felt (quickening), anatomy scan | | Anatomy Scan | Week 18–20 | Detailed ultrasound for fetal anatomy | | Viability | Week 22–24 | Extremely premature survival possible with intensive care | | Third Trimester | 27–40 | Rapid growth, lung maturation, birth preparation | | Full Term | 37–40 weeks | Normal delivery range | | Post Term | 42+ weeks | Induction often recommended |
Gestational age vs. fetal age: Gestational age counts from the LMP date — the first day of the last period, which is typically 2 weeks before conception. Fetal age counts from conception. This is why pregnancy is counted as 40 weeks from LMP even though actual conception is approximately 38 weeks before birth. Healthcare providers use gestational age; pregnancy apps and calculators all reference gestational age.
Frequently Asked Questions
How accurate is the due date calculated from LMP? Due dates from LMP are estimates — only about 4–5% of babies are born on their exact due date. About 80% of births occur within 2 weeks before or after the EDD. Ultrasound dating in the first trimester (ideally between 8–13 weeks, measuring crown-rump length) is more accurate than LMP dating, with a margin of error of ±5–7 days. If LMP and early ultrasound differ by more than 5–7 days, providers typically update the due date to match the ultrasound.
What is considered early, full term, and late term? Premature: before 37 weeks. Early term: 37–38 weeks 6 days. Full term: 39–40 weeks 6 days. Late term: 41–41 weeks 6 days. Post term: 42+ weeks. The "full term" definition was refined in 2013 — previously 37 weeks was considered full term, but research showed that 37–38 week babies have more complications than 39–40 week babies. Most providers now aim to avoid elective induction or C-section before 39 weeks without medical indication.
When should I have my first prenatal appointment? Most providers recommend the first prenatal visit between weeks 8–12. Earlier visits (before 8 weeks) are sometimes appropriate to confirm pregnancy, check for ectopic pregnancy symptoms, or for high-risk pregnancies. The first appointment typically includes: due date confirmation, medical history review, blood work (blood type, Rh factor, complete blood count, STI screening, immunity tests), urine testing, and sometimes the first ultrasound (dating scan).
Can the due date be wrong based on ultrasound? Ultrasound is very accurate in the first trimester for dating (±5–7 days) but becomes less accurate as pregnancy progresses. In the second trimester, ultrasound can be off by ±2 weeks; in the third trimester, ±3 weeks. This is because fetal size differences between pregnancies become more pronounced as growth varies by genetics and nutrition. The first-trimester ultrasound measurement is the gold standard for due date accuracy when available.
What happens if my pregnancy goes past the due date? Going past 40 weeks is very common — approximately 10–15% of pregnancies reach 42 weeks or beyond. At 41 weeks, most providers begin discussing induction. After 42 weeks, the placenta may begin to function less efficiently, and induction or C-section is typically recommended. During the post-date period, providers typically increase monitoring: non-stress tests, biophysical profiles, and amniotic fluid assessments to ensure the baby remains healthy.
Related Free Tools on RoughTools
- Ovulation Calculator — calculate fertile window and ovulation date
- Pregnancy Week Calculator — track pregnancy milestones week by week
- Water Intake Calculator — increased hydration needs during pregnancy
- Calorie Calculator — caloric needs increase during pregnancy (300–500 extra cal/day in trimesters 2–3)
Calculate Your Due Date Now
The free Pregnancy Calculator at RoughTools calculates your due date from LMP or conception date, shows current gestational age, and maps all key pregnancy milestones on a visual timeline. No account needed, completely free.