Pregnancy Due Date: How to Calculate Your Due Date and Track Pregnancy Week by Week

After a positive pregnancy test, one of the first questions most expectant parents ask is: When is my baby due? While the average pregnancy lasts about 40 weeks (280 days) from the first day of your last menstrual period, only about 5% of babies arrive exactly on their due date. Understanding how due dates are calculated helps you track each trimester and prepare for the big day.

The Standard Calculation Method

Healthcare providers typically calculate the due date using Naegele’s Rule:

1. Take the first day of your last menstrual period (LMP)
2. Add 280 days (40 weeks)
3. Alternatively: add 7 days, subtract 3 months, and add 1 year

For example, if your LMP was January 1, 2026, your estimated due date would be October 8, 2026. You can use our pregnancy calculator to get your exact due date and see a week-by-week breakdown.

First Trimester (Weeks 1–13)

During the first trimester, your body undergoes rapid changes. Key milestones include:

Week	Baby’s Development	Common Symptoms
4–5	Heart begins to beat	Fatigue, nausea
8–9	Fingers and toes form	Morning sickness peaks
12	All organs formed	Energy may return

Second Trimester (Weeks 14–27)

This is often called the “honeymoon phase.” Nausea typically subsides, and you may feel the baby move for the first time (quickening) around weeks 16–22. The second trimester anatomy scan (around week 20) checks the baby’s development and can often reveal the sex.

Third Trimester (Weeks 28–40)

As the baby grows, you may experience more physical discomfort. The third trimester is when you will have more frequent prenatal visits, glucose screening, and group B strep testing. Braxton Hicks contractions (practice contractions) may begin around week 32.

Why Due Dates Are Estimates

Only about 1 in 20 babies are born exactly on their due date. Most arrive within a window of 37 to 42 weeks. Factors that affect timing include:

• Cycle length — women with longer cycles may ovulate later
• First pregnancy — first babies tend to arrive slightly later
• Multiple babies — twins and triplets often arrive earlier
• Accuracy of LMP recall — if you are unsure of your LMP, an early ultrasound can refine the estimate

Track your pregnancy week by week with our pregnancy due date calculator to see your estimated due date, current week, and trimester breakdown.