Conception Date to Due Date Calculator — Pregnancy Length

266 Days from Conception to Birth

Full-term gestation from the actual moment of fertilization averages 266 days, which equals 38 weeks. This is distinct from the more commonly cited 280-day (40-week) pregnancy duration, which is measured from the first day of the last menstrual period (LMP). The 14-day difference accounts for the typical interval between the start of menstruation and ovulation in a standard 28-day cycle. Research from large pregnancy outcome studies, including data from the World Health Organization and the March of Dimes, confirms that 266 days from conception is the mean gestational age for spontaneous singleton pregnancies. The term 'full term' is defined as 37 to 42 weeks from LMP, or 35 to 40 weeks from conception.

The 40-Week Convention Explained

The 40-week (280-day) pregnancy convention, established by German obstetrician Franz Karl Naegele in 1812, counts from the first day of the last menstrual period rather than from conception. This was adopted because LMP is a concrete, observable date that most women can identify reliably, whereas the exact moment of conception is rarely known. The convention became the global standard for obstetric care, ultrasound dating, and pregnancy management. Today, all clinical pregnancy dating, ultrasound growth charts, and prenatal care milestones are expressed in weeks from LMP. When a doctor says you are 12 weeks pregnant, they mean 12 weeks from LMP, which is approximately 10 weeks from actual conception.

Calculating Due Date from Known Conception

If you know your exact conception date, add 266 days (38 weeks) to calculate your due date. This is most precise in assisted reproductive technology cases. For in vitro fertilization (IVF) with a day-3 embryo transfer, the equivalent gestational age is transfer date plus 17 days (2 weeks 5 days from LMP). For a day-5 blastocyst transfer, the equivalent is transfer date plus 19 days (2 weeks 7 days from LMP). For intrauterine insemination (IUI), conception is assumed to occur 12 to 24 hours after the procedure, so add 266 days from the IUI date. Your reproductive endocrinologist will provide the official gestational age calculation.

Naegele Rule vs. Conception-Based Calculation

The Naegele rule calculates due date from LMP using the formula: LMP plus 7 days minus 3 calendar months. For an LMP of January 1, the due date is October 8. This formula implicitly assumes a 28-day cycle and adds the standard 280 days of gestation. The conception-based formula calculates due date by adding 266 days to the known conception date, which is mathematically equivalent for a 28-day cycle but more accurate when the conception date is independently confirmed. For cycles longer than 28 days, the Naegele rule overestimates the due date unless adjusted by the number of days by which the cycle exceeds 28 days.

Due Date Accuracy: Only 5% Deliver On Time

The due date is an estimate, not a deadline. Only about 4 to 5% of babies are born on their exact due date. Approximately 80% are born within the 2-week window from 38 to 42 weeks from LMP. The remaining 20% are born before 38 weeks (preterm or early term) or after 42 weeks (post-term). Factors that influence actual delivery timing include whether this is a first pregnancy (first-time mothers average about 3 weeks later than the due date), fetal genetics, maternal health conditions, and previous pregnancy outcomes. Induction of labour is typically considered at 41 weeks for low-risk pregnancies and earlier if complications arise. The due date serves as the reference point for monitoring fetal growth and planning prenatal care rather than a predicted delivery date.