Pregnancy Due Date Calculator

Enter Your Last Menstrual Period

The 280-Day Countdown, Standardized

Your estimated due date (EDD) is calculated using Naegele's rule, one of the most widely used methods in obstetrics. The formula adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This method assumes a standard 28-day menstrual cycle with ovulation occurring around day 14. While it is a reliable estimate, only about 5% of babies arrive on their exact due date — most healthy pregnancies fall within a two-week window around the estimated date.

The reason the calculation starts from your LMP rather than the date of conception is that most women know when their last period began but the exact date of ovulation and fertilization can be difficult to pinpoint. In a typical 28-day cycle, conception usually occurs about two weeks after the LMP, which is why pregnancy is counted as 40 weeks from LMP even though the embryo has only been developing for about 38 weeks.

When Each Trimester Actually Begins

Pregnancy is divided into three trimesters, each lasting approximately 12 to 14 weeks. Each stage brings unique developmental milestones for your baby and physical changes for you.

• First Trimester (Weeks 1-12): Your baby's major organs begin forming. You may experience fatigue, nausea, and mood swings. The risk of miscarriage is highest during this period, but drops significantly after week 12.
• Second Trimester (Weeks 13-27): Often called the "golden trimester" as many early symptoms ease. Your baby grows rapidly, you may feel first movements (quickening), and the pregnancy becomes more visible. Use our Baby Feeding Calculator to start planning ahead.
• Third Trimester (Weeks 28-40): Your baby gains weight rapidly and prepares for birth. You may experience more discomfort as your baby grows. Your healthcare provider will monitor you more frequently during this period.

Why Due Dates Are Estimates, Not Guarantees

While a due date provides a helpful target, it is important to remember that it is an estimate, not a deadline. Only about 4% of babies arrive on their predicted due date. Many factors influence when labor begins, including genetics, baby's position, and whether this is your first pregnancy. About 57% of first-time mothers go past their due date while subsequent pregnancies tend to be slightly shorter.

Your healthcare provider will monitor your pregnancy closely as you approach and pass your due date. Non-stress tests, ultrasounds, and amniotic fluid checks help ensure your baby remains healthy. Induction is generally recommended between 41 and 42 weeks to reduce the risk of complications. For your baby's early health tracking, consider our Baby Feeding Calculator and Conception Date Calculator.

Why Due Dates Shift (and When They Matter)

Your due date is a moving estimate until it's been confirmed by an early scan. Here's why it can shift and when those shifts actually matter:

• The math behind the date. Naegele's rule adds 280 days to the first day of your last menstrual period, assuming a uniform 28-day cycle with ovulation on day 14. If your cycles run 31 days or your ovulation comes late, the LMP math quietly drifts from reality.
• First pregnancy vs. later ones. First-time pregnancies tend to run slightly longer on average — roughly 57% of first-time mothers deliver after their due date. Later pregnancies are, on average, a few days shorter, which is one reason "your due date" feels more accurate with child number two.
• When the scan takes over. If an early ultrasound (before 14 weeks, when fetal measurements track true gestational age most tightly) disagrees with your LMP-based date by several days, providers typically adopt the scan date, because ovulation timing is the source of most LMP error — not fetal growth.

None of this changes your care on a normal day — but it determines decision windows like when screening tests are scheduled and when a provider starts talking about post-term monitoring.

Frequently Asked Questions

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