Estimated Due Date Calculator
How Each Calculation Method Works
There are three main clinical methods for estimating a due date, each with different inputs and accuracy profiles. This calculator supports all three.
Last Menstrual Period (LMP)
The most widely used method. EDD = LMP + 280 days (40 weeks), based on Naegele's Rule. Adjusted for cycle lengths other than 28 days: each day difference shifts the EDD by one day.
Conception Date
Used when ovulation or conception date is known. EDD = Conception date + 266 days (38 weeks). The LMP equivalent is estimated by subtracting 14 days from conception date.
IVF Day 5 Blastocyst
EDD = Transfer date + 261 days. A Day 5 embryo is already 5 days post-fertilization, so 266 − 5 = 261 days are added to the transfer date.
IVF Day 3 Cleavage
EDD = Transfer date + 263 days. A Day 3 embryo is 3 days post-fertilization, so 266 − 3 = 263 days are added to the transfer date.
Which method is most accurate? First-trimester ultrasound (crown-rump length measured at 8–14 weeks) is the gold standard for due date confirmation — more accurate than any calendar-based method. LMP dating assumes a 28-day cycle with ovulation on day 14, which is not true for all women. If your cycle is irregular, the conception-date or ultrasound method gives a more reliable estimate.
Calculation Method Comparison
| Method | Formula | Best For | Accuracy |
|---|---|---|---|
| LMP (28-day cycle) | LMP + 280 days | Regular cycles | ±2 weeks |
| LMP (adjusted cycle) | LMP + 280 ± (cycle−28) | Irregular cycles | ±10–14 days |
| Conception Date | Conception + 266 days | Known ovulation | ±5–7 days |
| IVF Day 5 Transfer | Transfer + 261 days | IVF blastocyst | ±3–5 days |
| IVF Day 3 Transfer | Transfer + 263 days | IVF cleavage | ±3–5 days |
| 1st Tri Ultrasound | CRL measurement | Any pregnancy | ±5–7 days |
Understanding Your Trimester Timeline
Pregnancy is divided into three trimesters, each spanning roughly 13 weeks and marked by distinct developmental milestones. The divisions are based on gestational age — weeks counted from the first day of the last menstrual period.
Only about 5% of babies are born on their due date. About 70% of births occur within 10 days of the EDD. The EDD is the midpoint of a probability distribution — it is the most likely individual date, but delivery anywhere between 37 and 42 weeks is medically normal. Due dates become more accurate when confirmed by early ultrasound.
Frequently Asked Questions
How accurate is the LMP method for calculating due dates?
The LMP method assumes a 28-day menstrual cycle with ovulation occurring on day 14. For women with regular 28-day cycles, it is accurate to within ±2 weeks in most cases. However, it is less reliable for women with irregular cycles, very short or long cycles, or those who conceived shortly after stopping hormonal contraception — because the cycle length and ovulation timing may not match the standard assumption. First-trimester ultrasound measurement of crown-rump length is more accurate and should be used to confirm or adjust the LMP-based EDD.
Does cycle length affect the due date?
Yes. The standard Naegele's Rule assumes a 28-day cycle. If your cycle is consistently longer — say, 35 days — ovulation likely occurs around day 21 rather than day 14, meaning conception happened about 7 days later than the standard assumption. The EDD is therefore shifted 7 days later as well. This calculator adjusts automatically for cycle lengths from 21 to 35 days. If your cycle varies significantly each month, the adjustment is only approximate and ultrasound dating is recommended.
How is the IVF due date calculated differently?
With IVF, the exact age of the embryo at transfer is known. A Day 5 blastocyst is already 5 days post-fertilization when transferred, so the EDD is calculated as transfer date plus 261 days (266 minus 5). A Day 3 cleavage embryo is 3 days post-fertilization, so EDD equals transfer date plus 263 days. IVF-based due date calculations are among the most accurate calendar methods because the fertilization date is precisely known — though ultrasound confirmation at 8–12 weeks is still recommended.
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of the last menstrual period and is the standard measure used in clinical obstetrics and on ultrasound reports. Fetal age (also called embryonic age or conception age) is counted from the actual date of fertilization, which is approximately 2 weeks after the LMP. So a pregnancy at 10 weeks gestational age has a fetus that is approximately 8 weeks old. All due date calculations, trimester divisions, and screening schedules in obstetric care use gestational age, not fetal age.
When should I expect to go into labor relative to my due date?
Spontaneous labor can begin any time from 37 to 42 weeks of gestation and still be considered normal. The EDD marks 40 weeks — the statistical midpoint. About 70% of births occur within 10 days of the EDD, either before or after. If labor has not begun by 41–42 weeks, most OB-GYNs will discuss induction to reduce the risk of complications associated with post-term pregnancy, including placental insufficiency and meconium aspiration. Your provider will guide decisions based on your specific clinical picture.
