Due Date Calculator

The Due Date Calculator estimates the expected date of delivery (EDD) based on the first day of the last menstrual period (LMP) and menstrual cycle length. This is the most commonly used method for pregnancy dating, employed by obstetricians and midwives worldwide as the initial step in prenatal care. Accurate pregnancy dating is essential because it determines the timing of virtually every aspect of prenatal management.

The standard calculation assumes pregnancy lasts 280 days (40 weeks) from the first day of the LMP. This convention is based on the observation that ovulation and conception occur approximately 14 days after the start of menstruation in a typical 28-day cycle, making the actual gestational period about 266 days from conception. For women with cycles longer or shorter than 28 days, the due date is adjusted by adding or subtracting the difference from the standard 28-day assumption, since ovulation timing shifts accordingly.

Due date estimation is fundamental to prenatal care because gestational age determines the scheduling of screening tests, ultrasound examinations, and monitoring protocols. It also defines important clinical thresholds that guide obstetric management: early term (37-38 weeks), full term (39-40 weeks), late term (41 weeks), and post-term (42+ weeks). Each of these categories carries different management implications regarding the timing of delivery, fetal surveillance, and intervention decisions.

It is important to note that only approximately 5% of babies are born on their exact estimated due date. The normal range for delivery extends from 37 to 42 weeks, with most births occurring within 10 days of the EDD. First pregnancies tend to go slightly longer than subsequent ones. When there is a discrepancy between LMP-based dating and first-trimester ultrasound measurements of crown-rump length (CRL), ultrasound dating is generally preferred if the difference exceeds 5 to 7 days, as early ultrasound provides the most accurate assessment of gestational age.

This calculator also estimates the conception date and trimester boundaries, which are important milestones for prenatal care planning. The first trimester ends at approximately 13 weeks (91 days from LMP), a period during which major organ formation occurs and most birth defects originate. The second trimester ends at approximately 26 weeks (182 days), during which rapid fetal growth and the anatomy scan occur. The third trimester extends from 27 weeks to delivery and focuses on fetal maturation, lung development, and weight gain.

Naegele Rule, the traditional method for EDD calculation developed by German obstetrician Franz Karl Naegele in the early 19th century, adds 7 days to the LMP date and then adds 9 months (or equivalently subtracts 3 months). This calculator uses the mathematically equivalent approach of adding 280 days to the LMP with cycle length adjustment, which produces identical results while being computationally simpler.

For the most accurate pregnancy dating, the American College of Obstetricians and Gynecologists (ACOG) recommends that first-trimester ultrasound measurement of crown-rump length should be obtained between 8 and 13 weeks of gestation, especially when the LMP date is uncertain or the menstrual cycle is irregular. When ultrasound dating and LMP dating agree within expected parameters, the LMP-based date is typically used. This calculator serves as a convenient starting point for pregnancy planning and should be complemented by clinical evaluation.