Why is pregnancy 40 weeks if gestation is 9 months?

Medical professionals count pregnancy as 40 weeks (280 days) starting from the first day of your last period. Since ovulation occurs roughly 2 weeks after your period, the baby is actually only 38 weeks old at birth.

Can I change my due date later?

Yes. A first-trimester ultrasound is the most accurate way to date a pregnancy. If the ultrasound date differs by more than 7 days from your LMP date, your doctor may adjust your EDD.

Pregnancy Due Date Calculator — Tracking Your 40-Week Journey

When my sister was pregnant, we realized that different apps gave three different due dates. We built this engine to align with exact medical gestation cycles for absolute peace of mind.

Clinical Pregnancy Tracker

Naegele's Rule EDD Engine

Last Period Date (LMP)

Cycle Length (Days)

Enter your LMP to track your journey

1. Gestational Age vs. Fetal Age

There is often confusion about how "old" the baby is. Doctors use Gestational Age, which starts from the first day of your last period. This means for the first two weeks of your pregnancy, you aren't actually pregnant yet! Fetal Age is the actual age of the baby from the moment of conception, usually two weeks less than the gestational age.

2. Naegele's Rule: The Standard Formula

Most clinicians use Naegele's rule to estimate arrival. It assumes a 28-day cycle and a 280-day (40-week) gestation period.

EDD = LMP + 7 Days - 3 Months + 1 Year

3. Understanding Trimesters

First Trimester (0-13 Weeks): Rapid cell division and organ formation. The risk of miscarriage is highest here.
Second Trimester (14-27 Weeks): The "golden period." You begin to feel the baby move (Quickening) and the anatomy scan occurs around week 20.
Third Trimester (28-40+ Weeks): Final lung development and significant weight gain for the baby as it prepares for birth.

Trying to Conceive?

Track your most fertile days and optimize your path to parenthood.

Ovulation Tracker

Frequently Asked Questions (FAQ)

How accurate is a pregnancy due date? ▶

A due date is only an estimate. Only about 4% of babies are born on their exact due date. Most babies arrive between 37 and 42 weeks of pregnancy.

What is the most accurate way to calculate my due date? ▶

While calculating from your Last Menstrual Period (LMP) is the standard, an ultrasound in the first trimester (between 8 and 12 weeks) is considered the most accurate clinical method.

What if I have an irregular cycle? ▶

If your cycle is shorter or longer than the standard 28 days, you should adjust the calculation. Our calculator allows you to input your specific cycle length for a more precise estimation.

Does the due date change during pregnancy? ▶

Sometimes. After a 20-week anatomy scan, your doctor might adjust your due date if the baby is measuring significantly larger or smaller than expected, though first-trimester estimates are usually kept.

What is "Full Term"? ▶

A pregnancy is considered "full term" at 39 weeks. Babies born between 37 and 39 weeks are "early term," and those born at 41 weeks are "late term."