About

Estimating the date of delivery (EDD) is one of the first and most exciting steps in pregnancy. While a full-term pregnancy is traditionally cited as 40 weeks, accurate prediction depends on the length of your menstrual cycle. Standard calculators assume a perfect 28-day cycle, but variation is normal.

This tool uses an adjusted Naegele's Rule to provide a more personalized date. It accounts for cycles ranging from 20 to 45 days, shifting the ovulation and conception windows accordingly. Additionally, the 'Reverse Calculation' feature allows you to input a known Due Date to estimate when conception likely occurred, offering valuable insight for family planning and medical history.

Formulas

The calculator uses the Adjusted Naegele's Rule. The standard gestation is 280 days (40 weeks) from the LMP for a 28-day cycle.

EDD = LMP + 280 days + (CycleLength − 28)

For reverse calculation (finding Conception Date from Due Date), we subtract the standard gestational period of 266 days (38 weeks post-conception):

Conception ≈ EDD − 266 days

Reference Data

Method	Description	Accuracy
LMP (Naegele's Rule)	Add 7 days to LMP, subtract 3 months.	Standard initial estimate. Assumes 28-day cycle.
Adjusted LMP	LMP + 280 days + (Cycle Length - 28).	Better for irregular cycles.
Ultrasound (CRL)	Crown-Rump Length measurement in 1st trimester.	Most accurate (+/- 5 days).
Conception Date	Add 266 days (38 weeks) to conception.	Accurate if ovulation date is known (e.g., IVF).
Dating Scan	Performed at 8-14 weeks.	Gold standard for dating.
Fundal Height	Measuring belly size.	Less accurate, used for growth monitoring.

Frequently Asked Questions

Only about 4-5% of babies are born exactly on their estimated due date. Most healthy pregnancies deliver within a two-week window before or after that date (38 to 42 weeks).

Yes, but the accuracy decreases. If your cycle varies significantly (e.g., 25 days one month, 35 the next), it is best to use the average length of the last 3-6 cycles in the 'Cycle Length' field. An early dating ultrasound is recommended for better precision.

Reverse calculation is useful if you have been given a due date by a doctor (based on ultrasound) and want to know the approximate date of conception or the fertile window during which the pregnancy started.

Ovulation typically occurs 14 days before the *next* period. In a 28-day cycle, ovulation is on day 14. In a 35-day cycle, ovulation is on day 21. Since pregnancy begins at fertilization (ovulation), a longer cycle means the baby is 'younger' than the LMP suggests, pushing the due date later.