# Gestational Age Calculator

Calculate your gestational age in weeks and days based on your last menstrual period. See your trimester, days until due date, and baby's development.

## What this calculates

Gestational age is the standard way of describing how far along a pregnancy is. Enter the number of days since your last menstrual period to find out your gestational age in weeks and days, which trimester you are in, and what developmental milestones your baby is reaching.

## Inputs

- **Days Since Last Menstrual Period** (days) — min 0, max 300 — Number of days since the first day of your last menstrual period

## Outputs

- **Gestational Age** — formatted as text — Current gestational age in weeks and days
- **Trimester** — formatted as text — Current trimester of pregnancy
- **Days Until Due Date** — Estimated days remaining until 40 weeks
- **Development Stage** — formatted as text — Key developmental milestone at this gestational age

## Details

Gestational age is calculated from the first day of the last menstrual period (LMP), even though conception typically occurs about 2 weeks later. This convention means that on the day of conception, the gestational age is already approximately 2 weeks. A full-term pregnancy is 40 weeks (280 days) from the LMP.

The three trimesters divide the 40-week pregnancy into roughly equal periods. The first trimester (weeks 1-12) is when all major organs form. The second trimester (weeks 13-26) is characterized by rapid growth and the mother feeling fetal movement. The third trimester (weeks 27-40) focuses on fetal weight gain, lung maturation, and preparation for birth.

Gestational age is important for medical decisions including scheduling prenatal tests, assessing fetal growth, and planning delivery. Ultrasound measurements, especially in the first trimester, can provide a more accurate gestational age than LMP-based calculation if the menstrual cycle is irregular. This calculator uses the LMP method, which is the starting point for all pregnancy dating.

## Frequently Asked Questions

**Q: What is the difference between gestational age and fetal age?**

A: Gestational age is counted from the first day of the last menstrual period and is the standard used in clinical practice. Fetal age (also called conceptional age) is counted from the date of conception, which is typically about 2 weeks after the LMP. So fetal age is generally 2 weeks less than gestational age. When your doctor says you are 12 weeks pregnant, they mean 12 weeks gestational age, which is about 10 weeks since conception.

**Q: How accurate is LMP-based gestational age?**

A: LMP-based gestational age assumes a 28-day cycle with ovulation on day 14, which is not always the case. For women with irregular cycles, the LMP date may not accurately reflect gestational age. First-trimester ultrasound (crown-rump length measurement) is accurate to within 3-5 days and is considered the gold standard when there is a discrepancy of more than 7 days from the LMP estimate. Your healthcare provider may adjust your due date based on ultrasound findings.

**Q: What are the key milestones in each trimester?**

A: First trimester: heart begins beating at 6 weeks, all major organs form by 8 weeks, and the fetus is fully formed in miniature by 12 weeks. Second trimester: sex is visible on ultrasound around 18-20 weeks, the mother feels movement (quickening) at 16-22 weeks, and the fetus can hear sounds by 22-24 weeks. Third trimester: the lungs produce surfactant by 28-32 weeks, rapid weight gain occurs, and the baby reaches full maturity by 37-40 weeks.

**Q: When is a baby considered full-term?**

A: The American College of Obstetricians and Gynecologists defines term pregnancy as follows: Early term is 37 weeks 0 days through 38 weeks 6 days, Full term is 39 weeks 0 days through 40 weeks 6 days, Late term is 41 weeks 0 days through 41 weeks 6 days, and Post-term is 42 weeks 0 days and beyond. Babies born at full term (39-40 weeks) generally have the best outcomes. Elective delivery before 39 weeks is discouraged unless medically indicated.

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Source: https://vastcalc.com/calculators/health/gestational-age
Category: Health & Fitness
Last updated: 2026-04-21