Pregnancy Week Calculator
Calculate current pregnancy week and trimester
How to Use
- Choose your calculation method (last menstrual period or ultrasound)
- Enter the date of your last period OR ultrasound date with gestational age
- Click calculate to see your current pregnancy week
- View your trimester, due date, and days remaining
How Are Pregnancy Weeks Calculated?
Pregnancy weeks are calculated from the first day of your last menstrual period (LMP), not from conception. This method is used because the exact date of conception is often unknown, while the start of your last period is typically easier to remember.
Pregnancy is typically 40 weeks (280 days) from LMP, or 38 weeks from conception. When you're 4 weeks pregnant, for example, your baby was actually conceived about 2 weeks ago.
Understanding Pregnancy Trimesters
- First Trimester (Weeks 0-13): Embryo develops all major organs. Common symptoms include morning sickness, fatigue, and breast tenderness.
- Second Trimester (Weeks 14-27): Baby grows rapidly. You'll feel movements, energy often returns, and the 'baby bump' becomes visible.
- Third Trimester (Weeks 28-40): Baby gains weight and prepares for birth. You may experience frequent urination, back pain, and Braxton Hicks contractions.
Ultrasound Dating vs LMP Dating
Early ultrasound (performed between 7-13 weeks) is the most accurate method for pregnancy dating. It measures the baby's crown-rump length to estimate gestational age within 3-5 days of accuracy.
LMP dating assumes a regular 28-day cycle with ovulation on day 14. For women with irregular cycles, longer or shorter cycles, or uncertain LMP dates, ultrasound dating is preferred and will be used to adjust the due date.
Pregnancy Weeks vs Months
Healthcare providers track pregnancy in weeks because it's more precise than months. Here's the approximate conversion:
| Weeks | Months | Trimester |
|---|---|---|
| 1-4 | 1 month | First |
| 5-8 | 2 months | First |
| 9-13 | 3 months | First |
| 14-17 | 4 months | Second |
| 18-22 | 5 months | Second |
| 23-27 | 6 months | Second |
| 28-31 | 7 months | Third |
| 32-35 | 8 months | Third |
| 36-40 | 9 months | Third |
Frequently Asked Questions
- How many weeks pregnant am I?
- Count from the first day of your last menstrual period to today. Divide by 7 to get weeks. For example, if your LMP was 70 days ago, you're 10 weeks pregnant. Use this calculator for automatic calculation from either LMP or ultrasound data.
- Which is more accurate - LMP or ultrasound dating?
- Early ultrasound (7-13 weeks) is more accurate than LMP dating, especially if you have irregular cycles or are unsure of your LMP date. Ultrasound dating is accurate within 3-5 days, while LMP dating can vary by 1-2 weeks.
- Can my due date change?
- Yes. If an early ultrasound shows your baby is measuring more than 5-7 days different from the LMP-based due date, your doctor will typically adjust the due date based on the ultrasound measurement, which is considered more accurate.
- What if I don't know my LMP date?
- If you don't remember your LMP or have irregular cycles, your healthcare provider will use ultrasound dating. The first ultrasound will measure your baby to determine gestational age and estimate your due date. Use the ultrasound option in this calculator with your scan results.