The Complete Guide to Pregnancy Due Date Prediction

Few dates carry as much emotional weight as the due date. It anchors prenatal appointments, maternity leave, baby showers, and the quiet countdown that lasts nine months. Yet most people are surprised to learn that the due date is not a delivery date at all — it is an estimate built on a 250-year-old rule of thumb. This guide explains how the Pregnancy Due Date Predictor computes it, what the numbers actually mean, and when an ultrasound is the better answer.

The foundation is Naegele's rule, named for the German obstetrician Franz Karl Naegele, who published it in the early nineteenth century. The rule states that the estimated due date (EDD) is the first day of the last menstrual period (LMP) plus 280 days. Those 280 days — exactly 40 weeks — reflect the standard assumption that ovulation occurs about 14 days after the start of the period, followed by roughly 266 days of fetal development. Add the two, and you have the textbook nine-month timeline.

The calculator offers two entry points into that timeline. If you know the first day of your last menstrual period, it adds 280 days directly. If you know the conception date instead — because you were tracking ovulation, for example — it adds 266 days, the developmental interval from fertilization to birth. When you use the conception route, the page also derives the implied LMP by subtracting 14 days, so gestational age is always counted from the standard pregnancy starting point. The two paths converge on the same date when the cycle is the textbook 28 days.

Gestational age is a separate, equally important number. It is the amount of time elapsed since the LMP, expressed in completed weeks and days. A woman described as "16 weeks pregnant" has completed 16 full weeks and is in her 17th week; the milestone table on this page marks status in exactly those completed weeks. This is also why gestational age can be computed for any date — not just today — by simply changing the current date input, which is useful for planning scans or checking how far along you will be at a future appointment.

The trimesters divide the 40-week journey into three phases that the calculator reports automatically. The first trimester spans weeks 0 through 12, the period of organ formation and the time when early screening tests — the nuchal translucency scan around weeks 11 to 13 — take place. The second trimester runs from week 13 through week 27, featuring the anatomy scan near week 20. The third begins at week 28 and runs to term, the phase of fetal growth, glucose screening, and weekly visits in the final stretch.

It is worth emphasizing how much uncertainty sits inside the estimate. Only about 5 percent of babies arrive on their exact due date, and a delivery any time between 37 and 42 weeks is considered normal. The two biggest sources of error are the assumption of a 28-day cycle — many cycles are longer or shorter — and imperfect recall of the LMP. When the cycle is irregular or the LMP is unknown, obstetric guidelines recommend an early ultrasound, which measures the crown-rump length and typically refines the dating estimate to within a few days.

The milestones table on the calculator turns the abstract timeline into concrete checkpoints. It lists when a home test is most likely to read positive, when a heartbeat is typically first seen, when the first-trimester screening window closes, when the anatomy scan occurs, when viability is generally reached, and when the baby is considered full term. Each row is marked "Reached" or "Upcoming" based on today's gestational age, so the page doubles as a quick orientation of where you stand in the pregnancy.

Using the tool is deliberately simple: choose LMP or conception, enter the date, and confirm the current date. The results show the estimated due date with its weekday, the gestational age in weeks and days, the current trimester, and the days remaining. The copy button saves the whole summary for sharing with a partner or a care team. Everything runs entirely in your browser, so no medical information is ever transmitted to a server.

The due date is a planning tool, not a prophecy. Use it to schedule scans, plan leave, and pace the milestones — but treat it as a window, not a deadline. When your care provider performs an early ultrasound, let that measurement become the authoritative date, because it reflects your baby's actual size rather than a calendar assumption. Naegele's rule is a remarkable legacy that still anchors prenatal care; the Pregnancy Due Date Predictor simply makes it instant, clear, and private.