Pregnancy Due Date Calculator: How to Estimate Your Due Date and Track Each Trimester

After a positive pregnancy test, the first question most expectant parents ask is: “When is my due date?” 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. Our Pregnancy Due Date Calculator gives you a reliable estimate, but understanding how due dates work helps you plan each stage with confidence.

How Due Dates Are Calculated

The standard method uses Nägele’s Rule: take the first day of your last menstrual period (LMP), add 280 days (40 weeks). This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the due date shifts accordingly. Ultrasound measurements in the first trimester provide the most accurate estimates — crown-rump length measurements can predict due dates within 5–7 days.

Pregnancy Timeline: Week by Week

Trimester	Weeks	Key Milestones	Common Symptoms
First	1–12	Conception, heartbeat detected (~week 6), embryo to fetus transition, neural tube closes	Nausea, fatigue, breast tenderness, food aversions
Second	13–27	Quickening (first movements felt), anatomy scan (week 20), baby can hear sounds	Energy returns, baby bump visible, round ligament pain
Third	28–40	Baby gains weight rapidly, lungs mature, head engages in pelvis, Braxton Hicks contractions	Back pain, frequent urination, swelling, difficulty sleeping

Use our Pregnancy Due Date Calculator to see exactly which week you are in and what milestones to expect.

Factors That Can Shift Your Due Date

• Cycle length: A 35-day cycle shifts the due date about 7 days later than the standard 28-day assumption
• First ultrasound: If your first-trimester ultrasound measurements differ from your LMP calculation by more than 7 days, the ultrasound date is usually considered more accurate
• Multiple pregnancy: Twins and triplets tend to arrive earlier — the average for twins is 36 weeks, for triplets 32 weeks
• Previous pregnancies: First-time mothers tend to go slightly longer than those who have given birth before

Due Date Accuracy: What the Research Says

A full-term pregnancy is defined as 39 weeks 0 days to 40 weeks 6 days. Early-term is 37–38 weeks, late-term is 41 weeks, and post-term is 42+ weeks. Only about 4% of births occur on the exact due date. Most fall within a window of 37 to 42 weeks, which is why healthcare providers monitor progress rather than fixating on a single date. Use our Pregnancy Due Date Calculator to get a personalized timeline and track your progress week by week.