Due Date Calculator

How Are Due Dates Calculated?

The most common method is Naegele's rule, developed by German obstetrician Franz Naegele in the 19th century. It adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14.

If your cycle is longer or shorter than 28 days, the calculation should be adjusted. For example, a 35-day cycle means you likely ovulated around day 21 instead of day 14, so your due date would be about 7 days later than the standard calculation. This calculator makes that adjustment automatically.

Trimester Overview

• First trimester (weeks 1–12) — Major organ development begins. Early symptoms include nausea, fatigue, and breast tenderness. The first heartbeat is often detectable around week 6.
• Second trimester (weeks 13–27) — Often called the most comfortable trimester. The anatomy scan is typically done around week 20. You may start feeling movement around weeks 16–25.
• Third trimester (weeks 28–40) — Baby gains weight rapidly. Week 37 marks full term. Many providers recommend increased monitoring in the final weeks.

How Accurate Is a Due Date?

Only about 4–5% of babies arrive on their exact due date. Most deliveries occur between 38 and 42 weeks. A first-trimester ultrasound is typically accurate to within 5–7 days and is considered more reliable than LMP-based dating, especially for people with irregular cycles. Your healthcare provider may adjust your due date based on ultrasound findings.