Pregnancy Weeks to Months Calculator

How to Use This Calculator

1. Enter your weeks pregnant (1-42) to get months, trimester, and ACOG classification.
2. Use the Months → Weeks tab to convert in the opposite direction.
3. Use the Trimester Guide tab to see your progress percentage and weeks remaining.
4. Use the Professional tab with additional days input for exact gestational age in weeks+days format.

Formula

Example

Frequently Asked Questions

• How many months is 40 weeks of pregnancy? ▼
  A full-term pregnancy of 40 weeks equals approximately 9.2 calendar months (40 ÷ 4.345 weeks/month). This is why pregnancy is said to last "9 months" — though it is technically closer to 10 lunar months (4-week months) or 9.2 calendar months.
• How many weeks is each trimester? ▼
  Per standard obstetric convention: 1st trimester = weeks 1-12. 2nd trimester = weeks 13-27. 3rd trimester = weeks 28 through delivery (typically 40-42 weeks). Each trimester is approximately 13 weeks.
• What is the difference between gestational age and fetal age? ▼
  Gestational age is counted from the first day of the LMP and is how weeks of pregnancy are clinically measured — it includes ~2 weeks before conception. Fetal (fertilization) age is approximately 2 weeks less than gestational age.
• What is a full-term pregnancy according to ACOG? ▼
  Per ACOG 2013: Early term = 37-38 weeks 6 days; Full term = 39-40 weeks 6 days; Late term = 41 weeks; Post-term = ≥42 weeks. Before 37 weeks is preterm.
• Why does pregnancy seem to be "10 months"? ▼
  The "10 months" framing uses lunar months of exactly 28 days. 40 weeks ÷ 4 weeks = 10 lunar months. Calendar months average 30.44 days, so 40 weeks = 9.2 calendar months. Most people say "9 months" colloquially.

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