Due Date Calculator — Free 2026

Understanding Your Pregnancy Due Date

Finding out when your baby is expected to arrive is one of the most exciting moments of pregnancy. A due date calculator uses well-established medical formulas to provide an estimated delivery date (EDD) based on the first day of your last menstrual period (LMP). While the exact day your baby will arrive is impossible to predict with certainty, having an estimated due date is essential for planning prenatal care, scheduling tests, and preparing for your new arrival.

The most widely used method for calculating a due date is Naegele's Rule, developed by German obstetrician Franz Karl Naegele in the early 19th century. Despite being over 200 years old, this formula remains the standard starting point used by obstetricians, midwives, and pregnancy apps worldwide. The formula works by adding 280 days (40 weeks) to the first day of the last menstrual period. This assumes a standard 28-day menstrual cycle with ovulation occurring on day 14. If your cycle length differs from 28 days, the calculation adjusts accordingly — a longer cycle pushes the due date later, while a shorter cycle brings it earlier.

How Naegele's Rule Works

The mathematics behind Naegele's Rule is straightforward. Starting from the first day of your LMP, the formula adds 280 days to arrive at the estimated due date. An alternative way to think about it: take the LMP date, add one year, subtract three months, and add seven days. Both approaches yield the same result for a 28-day cycle. When the cycle length differs from 28 days, the adjustment is simple — add or subtract the difference. For example, if your cycle is 32 days, add 4 extra days to the standard 280-day calculation, giving you 284 days from the LMP.

It is important to understand that conception does not typically occur on the first day of the menstrual period. Ovulation usually happens about 14 days before the start of the next period. In a 28-day cycle, that means ovulation occurs around day 14. Since sperm can survive for up to five days and an egg remains viable for about 24 hours, the fertile window spans roughly six days per cycle. The conception date displayed by this calculator is an approximation based on estimated ovulation timing.

The Three Trimesters

Pregnancy is divided into three trimesters, each approximately 13 weeks long. The first trimester (weeks 1 through 12) is a period of rapid development. By the end of week 12, all major organs and body systems have begun to form. This trimester is associated with common symptoms like morning sickness, fatigue, and breast tenderness. It is also the period when the risk of miscarriage is highest, which is why many people wait until the second trimester to share their pregnancy news.

The second trimester (weeks 13 through 27) is often described as the most comfortable phase of pregnancy. Many of the early symptoms subside, and most expectant parents begin to feel the baby's movements — a milestone known as quickening — typically between weeks 18 and 25. The anatomy scan ultrasound, usually performed around week 20, provides a detailed look at the baby's development and can often reveal the sex. During this trimester, the baby grows from about 3 inches to approximately 14 inches in length.

The third trimester (weeks 28 through 40) is the home stretch. The baby gains significant weight, the lungs continue to mature, and the body prepares for labour and delivery. Common symptoms during this period include back pain, difficulty sleeping, Braxton Hicks contractions, and increased urinary frequency. A full-term pregnancy is considered to be between 39 and 40 weeks, though healthy deliveries occur anywhere from 37 to 42 weeks. Only about 5% of babies arrive on their exact due date — the majority are born within a two-week window on either side of the EDD.

Factors That Affect Your Due Date

Several factors can influence how accurate a due-date estimate turns out to be. Irregular menstrual cycles make it harder to pinpoint ovulation, which can shift the calculation. First-trimester ultrasound measurements — particularly crown-rump length — are considered the most accurate dating method and may lead your healthcare provider to adjust the LMP-based due date. Maternal age, health conditions such as gestational diabetes or preeclampsia, and whether the pregnancy involves multiples (twins, triplets) can all affect the actual timing of delivery.

If you are interested in tracking other health metrics during your pregnancy, our BMI calculator can help you understand your pre-pregnancy body mass index, which is one factor healthcare providers consider when setting weight gain goals. You might also find our age calculator useful for understanding maternal age, which is an important consideration in prenatal care planning.