EDD Calculator: Estimated Date of Delivery
Enter your last menstrual period, an ultrasound gestational age, your IVF transfer date, or a known conception date and this calculator returns your estimated due date, current gestational age, all three trimester dates, and key milestones. Results update instantly and include a step-by-step explanation of the math. This tool is for educational purposes only - always confirm your due date with a healthcare provider.
Formula
Worked example
LMP = January 1, 2025, cycle length 28 days: add 280 days to get October 8, 2025 (EDD). Cycle of 32 days: add 4 extra days, EDD = October 12, 2025. Ultrasound on Feb 14 at 10 weeks 0 days: 280 - 70 = 210 remaining days, EDD = Sep 12, 2025. IVF Day-5 blastocyst transfer on Jan 10: conception ~ Jan 5, EDD ~ Oct 28, 2025.
What is an EDD and how is it calculated?
The Estimated Date of Delivery (EDD), sometimes called the estimated due date (EDD) or simply the "due date," is the calendar date when a pregnancy is expected to reach 40 completed gestational weeks. Gestational age is counted from the first day of the last menstrual period (LMP), not from conception, so the full term pregnancy is 280 days (40 weeks) from LMP even though conception normally occurs about 14 days later.
The standard formula is Naegele's rule: add 280 days to the LMP date. If your cycle is longer or shorter than 28 days, the formula adjusts by the difference: a 32-day cycle shifts the EDD four days later; a 24-day cycle shifts it four days earlier. Ultrasound dating, IVF transfer records, and a known conception date all provide alternative anchors for the same arithmetic, with ultrasound in the first trimester being clinically the most precise.
Only about 5% of babies are born on their EDD. A delivery is considered full term anywhere from 39 to 40 weeks 6 days, early term from 37 to 38 weeks 6 days, and post-term at 42 weeks or beyond.
Four calculation methods explained
Last Menstrual Period (LMP): The most commonly used method. Enter the first day of your last period and your average cycle length. The calculator applies Naegele's rule with a cycle-length correction: EDD = LMP + 280 + (cycle length - 28) days. Most accurate when you know your LMP precisely and have regular cycles.
Ultrasound gestational age: When your provider measured a gestational age during a scan, enter the scan date and that measurement. The calculator works backwards: remaining days = 280 - GA at scan, then adds those days to the scan date. First-trimester ultrasound (using crown-rump length, typically at 7-14 weeks) is the most accurate single dating method and may revise an LMP-based EDD.
IVF / embryo transfer: For IVF pregnancies, the exact age of the embryo at transfer is known. Conception is reconstructed as: transfer date minus embryo age (day 3, 5, or 6). EDD = conception date + 266 days. This method is extremely precise when the transfer date and embryo day are documented accurately.
Conception date: If you know the approximate date of unprotected sex or ovulation testing that led to conception, enter it directly. EDD = conception date + 266 days (38 weeks from fertilisation to term). The LMP is estimated as 14 days before conception.
Understanding trimesters and key milestones
Pregnancy is divided into three trimesters:
- First trimester (weeks 1-13): Covers organogenesis and the highest-risk period for miscarriage. Key events include the first heartbeat (~week 6), first-trimester screening (weeks 11-14 for nuchal translucency + blood tests), and the CVS procedure window (weeks 10-13).
- Second trimester (weeks 14-27): Often the most comfortable trimester. The anatomy scan (18-20 weeks) checks fetal development in detail and can often reveal biological sex if desired. The viability milestone falls at 24 weeks, after which survival with intensive neonatal care becomes possible.
- Third trimester (weeks 28-40+): Rapid fetal weight gain. Routine checks include glucose screening (24-28 weeks), Group B Strep swab (35-37 weeks), and ongoing fetal movement monitoring. Early term begins at 37 weeks, full term at 39 weeks, and post-term at 42 weeks - when additional monitoring or induction is typically discussed.
How accurate is an estimated due date?
The EDD is a statistical midpoint, not a precise prediction. Research shows that the standard deviation around a due date is roughly 10-13 days for spontaneous labour onset, meaning roughly two-thirds of deliveries fall within about two weeks of the EDD in either direction. Clinical factors that can shift the EDD include:
- Irregular or unknown menstrual cycles (LMP method becomes less reliable)
- Late presentation for prenatal care (first-trimester ultrasound is most accurate; accuracy decreases as gestation advances)
- Multiple gestation - twins and higher-order multiples typically deliver earlier (twins average around 35-36 weeks)
- Conditions such as PCOS that affect ovulation timing
Key pregnancy milestones by gestational age
| Week | Milestone | Notes |
|---|---|---|
| 1-4 | Conception and implantation | Fertilisation typically occurs around week 2 |
| 6-8 | First heartbeat detectable | Transvaginal ultrasound can confirm cardiac activity |
| 10-13 | First trimester screening | Nuchal translucency scan + blood tests for chromosomal conditions |
| 14 | Second trimester begins | Miscarriage risk drops significantly after week 12-14 |
| 18-20 | Anatomy scan | Detailed fetal survey; sex may be visible if desired |
| 24 | Viability milestone | Survival with intensive neonatal care becomes possible |
| 28 | Third trimester begins | Glucose screening, Group B Strep test later in trimester |
| 34-36 | Late preterm | Delivery may occur; NICU support often needed |
| 37 | Early term | Lungs largely mature; elective delivery generally acceptable |
| 39 | Full term | Optimal timing for uncomplicated deliveries |
| 40 | Due date | Naegele rule target; ~80% of deliveries occur within 2 weeks of EDD |
| 41 | Late term | Provider may discuss induction options |
| 42+ | Post-term | Increased monitoring recommended; induction often advised |
Gestational age is counted from the first day of the last menstrual period (LMP).
Frequently asked questions
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of your last menstrual period (LMP), so it includes roughly two weeks before conception occurred. Fetal age (also called conceptional age) starts from fertilisation and is therefore about two weeks less than gestational age. Healthcare providers almost always use gestational age because LMP is easier to recall than the exact conception date, and all standard dating charts are built on that convention.
Can my due date change after an ultrasound?
Yes. If a first-trimester ultrasound (up to 13 weeks 6 days) produces an EDD that differs from the LMP-based EDD by more than 7 days, ACOG guidelines recommend updating the EDD to the ultrasound estimate. Ultrasound accuracy decreases as pregnancy advances, so revisions are less common after the first trimester and very rare after 20 weeks.
Is 40 weeks the right target for my due date?
Forty weeks (280 days from LMP) is the statistical average for singleton pregnancies in populations that informed Naegele's rule. Some studies suggest the true average for spontaneous labour may be slightly longer (around 40 weeks 3-5 days). The "full term" window defined by ACOG is 39 weeks 0 days to 40 weeks 6 days; the best outcomes for uncomplicated pregnancies cluster in this range.
How is the due date calculated for IVF pregnancies?
For IVF, the exact embryo age at transfer is known, which removes the uncertainty around ovulation timing. A day-5 blastocyst was fertilised 5 days before transfer, so conception is transfer date minus 5. Adding 266 days (38 weeks) from conception gives the EDD. Some clinics also use a simpler convention of transfer date + 261 days for day-5 transfers, which produces the same result.
What does "viability at 24 weeks" mean?
The 24-week viability milestone is the gestational age at which survival outside the womb becomes possible with specialised neonatal intensive care. Survival rates improve rapidly with each additional week of gestation after 24 weeks, but outcomes at 24-25 weeks vary greatly and depend on the level of neonatal support available. This milestone does not mean delivery at 24 weeks is safe or desirable, only that intensive intervention may be appropriate.
What is the difference between early term, full term, and post-term?
These are ACOG categories: early term is 37 weeks 0 days to 38 weeks 6 days, full term is 39 weeks 0 days to 40 weeks 6 days, late term is 41 weeks 0 days to 41 weeks 6 days, and post-term is 42 weeks 0 days or beyond. Elective deliveries before 39 weeks carry higher risks of respiratory and feeding problems even though the baby is technically "term," which is why the full-term window matters clinically.
My cycle is not 28 days. How does that affect my due date?
Each day your cycle differs from 28 days shifts your estimated due date by one day. A 35-day cycle means ovulation likely occurred around day 21 rather than day 14, so the EDD is 7 days later than the standard Naegele calculation. This calculator applies that correction automatically. If your cycles are highly irregular, a first-trimester ultrasound is the most reliable way to establish your EDD.
Sources
- American College of Obstetricians and Gynecologists. "Methods for Estimating the Due Date." ACOG Committee Opinion No. 700, 2017.
- Naegele, F.C. Erfahrungen und Abhandlungen aus dem Gebiete der Krankheiten des weiblichen Geschlechtsorganes. Heidelberg, 1812. (Original description of Naegele's rule.)
- Mongelli M, Wilcox M, Gardosi J. "Estimating the date of confinement: ultrasonographic biometry versus certain menstrual dates." American Journal of Obstetrics and Gynecology, 1996.