Health & Medicine · Obstetrics & Pediatrics
Pregnancy Due Date Calculator
Calculate your estimated due date (EDD) using Naegele's Rule based on the first day of your last menstrual period or conception date.
Calculator
Format: YYYY-MM-DD
Formula
EDD is the Estimated Due Date. LMP is the first day of the Last Menstrual Period. 280 days equals 40 weeks, which is the standard gestational length based on a 28-day cycle. Naegele's Rule equivalently adds 1 year, subtracts 3 months, and adds 7 days to the LMP date.
Source: Naegele, F.C. (1812). Erfahrungen und Abhandlungen aus dem Gebiete der Krankheiten des weiblichen Geschlechtes. ACOG Practice Bulletin No. 700, American College of Obstetricians and Gynecologists, 2017.
How it works
Naegele's Rule, formulated by German obstetrician Franz Karl Naegele in 1812, remains the standard method for calculating an estimated due date (EDD). The rule is elegantly simple: add 280 days (40 weeks) to the first day of the last menstrual period. This is because human pregnancy is conventionally measured from the LMP, which typically precedes actual conception by approximately 14 days in a standard 28-day cycle. Therefore, 40 weeks of gestational age includes roughly 2 weeks before conception occurred.
The 280-day figure represents the average length of human gestation derived from population-level obstetric data. However, not every woman has a 28-day cycle. Women with longer cycles (e.g., 35 days) tend to ovulate later, effectively shifting the due date forward, while those with shorter cycles (e.g., 21 days) may ovulate earlier, shifting the EDD back. This calculator adjusts the raw 280-day estimate by adding or subtracting the difference between the actual cycle length and the standard 28 days, yielding a more individualized prediction.
In clinical practice, the EDD established by LMP is often confirmed or revised by first-trimester ultrasound, which measures fetal crown-rump length (CRL). The American College of Obstetricians and Gynecologists (ACOG) recommends that if the ultrasound EDD differs from the LMP-based EDD by more than 5–7 days in the first trimester, the ultrasound date should take precedence. Gestational age is typically expressed in weeks and days (e.g., 12 weeks and 3 days), and the normal range for full-term delivery spans from 39 weeks 0 days to 40 weeks 6 days, with 37 to 41 weeks considered term.
Worked example
Consider a woman whose LMP began on January 15, 2025, with a cycle length of 30 days.
Step 1 — Base EDD using 280 days: January 15, 2025 + 280 days = October 22, 2025.
Step 2 — Cycle length adjustment: Her cycle is 30 days, which is 2 days longer than the standard 28-day cycle. Therefore, we add 2 days to the base EDD: October 22 + 2 days = October 24, 2025.
Step 3 — Gestational age on April 1, 2025: From January 15 to April 1 is 75 days, which equals 10 weeks and 5 days of gestational age. This falls in the First Trimester.
Step 4 — Days remaining: From April 1 to October 24, 2025 is approximately 206 days, or about 29 weeks until the estimated due date.
This EDD is an estimate. The baby is considered full term if born between 39 weeks 0 days and 40 weeks 6 days, and term if born between 37 and 41 weeks.
Limitations & notes
Naegele's Rule assumes a regular 28-day menstrual cycle with ovulation on day 14, which does not apply to all women. Irregular cycles, polycystic ovary syndrome (PCOS), recent hormonal contraceptive use, or breastfeeding can all affect ovulation timing and make the LMP-based estimate less reliable. Studies have shown that even in ideal conditions, only about 4–5% of babies are born on their exact calculated due date, and the normal distribution of birth dates spans several weeks around the EDD. First-trimester ultrasound dating, particularly between 8 and 13 weeks using crown-rump length measurement, is considered more accurate than LMP-based calculations and is the preferred method when the LMP is uncertain or cycles are irregular. This calculator is intended for informational and educational purposes only and does not constitute medical advice. Always confirm your due date with a qualified healthcare provider.
Frequently asked questions
How accurate is the estimated due date from Naegele's Rule?
Naegele's Rule provides a statistically useful estimate, but only about 4–5% of babies are born exactly on their due date. Research published in the British Journal of Obstetrics and Gynaecology found that the average spontaneous birth occurs at approximately 40 weeks and 5 days for first-time mothers and 40 weeks and 3 days for subsequent pregnancies. The EDD is best understood as the center of a normal distribution rather than a precise deadline.
What is the difference between gestational age and fetal age?
Gestational age is measured from the first day of the last menstrual period (LMP) and is the standard clinical measure used by doctors and midwives — it includes approximately 2 weeks before conception actually occurred. Fetal age (also called conceptional or embryonic age) is measured from the date of actual fertilization and is typically about 2 weeks less than gestational age. When a doctor says you are '10 weeks pregnant,' they mean 10 weeks of gestational age, not 10 weeks since conception.
What are the three trimesters of pregnancy?
Pregnancy is divided into three trimesters: the First Trimester spans from week 1 to the end of week 13 (gestational days 1–97), during which all major organs form and the risk of miscarriage is highest; the Second Trimester covers weeks 14 through 27 (days 98–195), a period of rapid fetal growth when most women feel their best; and the Third Trimester extends from week 28 until delivery (day 196 onward), characterized by continued fetal maturation and preparation for birth.
Can I use this calculator if I know my conception date instead of my LMP?
Yes. If you know your conception date, you can estimate your LMP by subtracting 14 days (for a standard 28-day cycle) and enter that as your LMP date with a cycle length of 28 days. Alternatively, add 266 days (38 weeks) directly to your conception date to estimate the EDD, since conception typically occurs 14 days into the cycle and gestation from conception is approximately 38 weeks. Conception-based dating is useful for women who underwent IVF or who tracked ovulation closely.
When should ultrasound dating override the LMP-based due date?
ACOG guidelines recommend that first-trimester ultrasound (performed between 8 and 13 weeks 6 days) should be used to confirm or revise the EDD. If the ultrasound-based EDD differs from the LMP-based EDD by more than 5 days before 9 weeks, more than 7 days between 9 and 15 weeks 6 days, or more than 10 days between 16 and 21 weeks 6 days, the ultrasound date should take precedence. Ultrasound dating is especially important when cycles are irregular, the LMP is uncertain, or the patient was recently using hormonal contraception.
Last updated: 2025-01-15 · Formula verified against primary sources.