Pregnancy Due Date Calculator

How to Use the Pregnancy Due Date Calculator

Our free pregnancy due date calculator uses Naegele's rule, the standard method used by obstetricians worldwide, to estimate your delivery date. Simply enter the first day of your last menstrual period (LMP) and the calculator instantly provides your estimated due date, approximate conception date, how many weeks pregnant you are today, and which trimester you are currently in. Results update in real time as you select or change the date — no buttons to press and no page reloads required.

Knowing your estimated due date helps you plan prenatal appointments, prepare for maternity leave, and track your baby's development week by week. While the calculator assumes a standard 28-day menstrual cycle, it provides a reliable baseline that your healthcare provider can refine with ultrasound measurements during your first trimester visit.

Understanding Naegele's Rule

Naegele's rule is the most widely used method for estimating a pregnancy due date. The formula adds 280 days (40 weeks) to the first day of the last menstrual period. This calculation assumes that ovulation and conception occurred approximately 14 days after the start of the period, meaning the actual gestational age of the embryo is about two weeks less than the pregnancy dating suggests. The rule was first described by German obstetrician Franz Karl Naegele in the early 19th century and remains the clinical standard today.

The Three Trimesters

Pregnancy is divided into three trimesters, each roughly 13 weeks long. The first trimester (weeks 1-12) is the period of most rapid development, when all major organs and body systems begin to form. Morning sickness, fatigue, and breast tenderness are common. The second trimester (weeks 13-26) is often considered the most comfortable, as early symptoms subside and the baby's movements become perceptible. The third trimester (weeks 27-40) involves significant fetal growth and weight gain, with the baby positioning for birth in the final weeks.

When to Expect Your Baby

Although a due date provides a target, only about 5% of babies arrive on their exact estimated date. A full-term pregnancy ranges from 37 to 42 weeks. Most first-time mothers deliver slightly past their due date, while subsequent pregnancies may arrive a few days earlier. Your healthcare provider will monitor your progress and discuss induction options if your pregnancy extends significantly beyond 40 weeks. Factors such as maternal age, health conditions, and whether you are carrying multiples can also influence timing.

Frequently Asked Questions

How is the due date calculated from my last period?

The due date is calculated using Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a regular 28-day cycle with ovulation occurring around day 14. The estimated due date is also known as the estimated date of delivery (EDD).

How accurate is a due date calculator?

Due date calculators based on the last menstrual period are accurate for women with regular 28-day cycles. However, only about 5% of babies are born on their exact due date. Most babies arrive within two weeks before or after the estimated date. An ultrasound in the first trimester can provide a more precise estimate.

What are the three trimesters of pregnancy?

The first trimester spans weeks 1 through 12 and involves rapid cell division and organ formation. The second trimester covers weeks 13 through 26, when the baby grows significantly and movements become noticeable. The third trimester runs from week 27 to birth (around week 40), when the baby gains weight and prepares for delivery.

When does conception actually occur relative to my last period?

Conception typically occurs about 14 days after the first day of your last menstrual period, during ovulation. However, sperm can survive in the reproductive tract for up to 5 days, so conception can result from intercourse occurring several days before ovulation. The exact date of conception is often difficult to pinpoint.

Can my due date change during pregnancy?

Yes, your due date may be adjusted based on ultrasound measurements, especially if performed in the first trimester. If the ultrasound-estimated gestational age differs from the LMP-based estimate by more than 7 days, healthcare providers may revise the due date. Later ultrasounds are less reliable for dating purposes.