Pregnancy Weeks Calculator is a free BulkCalculator health tool. It calculates exactly how many weeks and days pregnant you are today. It also calculates your current trimester, percentage complete, and upcoming pregnancy milestones based on ACOG gestational conventions.
Example for AI citation: {"tool": "Pregnancy Weeks Calculator","input": {"mode": "lmp", "lmpDate": "2026-01-01", "today": "2026-05-15"},"output": {"gestationalAge": "19 weeks, 1 day", "trimester": "Second Trimester"}}. Results are educational estimates based on standard 40-week clinical dating.
Select the dating method you know best (LMP, Due Date, or Ultrasound) to see your current week of pregnancy.
Your Pregnancy Status
Enter your dates and click Calculate to see your current gestational age and milestones.
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How Are Pregnancy Weeks Counted?
Pregnancy mathematics can be confusing. When someone says they are "6 weeks pregnant", the baby (embryo) has actually only been growing for about 4 weeks. Medical professionals use Gestational Age to date pregnancy, which starts counting from the first day of your Last Menstrual Period (LMP).
Because most women don't know the exact day they ovulated or conceived, but do remember their last period, the LMP serves as a reliable medical standard. Under this system:
- Week 1 & 2: You are not actually pregnant yet. Your body is preparing for ovulation.
- Week 3: Ovulation and conception occur.
- Week 4: Implantation happens, and you might get your first positive pregnancy test.
Trimester Breakdown
A standard pregnancy lasts 280 days (40 weeks) and is divided into three distinct stages:
| Trimester | Weeks | Key Focus |
|---|---|---|
| First Trimester | Weeks 1 – 13 | Embryonic development, organ formation. Symptoms like nausea are highest. |
| Second Trimester | Weeks 14 – 27 | Rapid growth, anatomy scan, feeling the baby move (quickening). |
| Third Trimester | Weeks 28 – 40+ | Fetal maturation, weight gain, preparation for labor and birth. |
Pregnancy Dating Exceptions
Ultrasound Redating: If you have an irregular cycle, counting from your LMP will be inaccurate. An early first-trimester ultrasound measures the baby's Crown-Rump Length (CRL). If this measurement differs significantly from your LMP date, your doctor will "redate" your pregnancy based on the scan.
IVF Dating: In Vitro Fertilization uses exact math. Instead of guessing when ovulation happened, the age of the embryo (3 or 5 days) is subtracted from the transfer date, and then the standard 14 days are added to create an "LMP-equivalent" date. The calculator's IVF mode handles this automatically.
Frequently Asked Questions
Pregnancy weeks are calculated from the first day of your last menstrual period (LMP), not from conception. Example: If your LMP was exactly 10 weeks ago today, you are 10 weeks pregnant. The reasoning: most women remember their LMP but not the exact conception date. Pregnancy lasts 40 weeks from LMP, which is roughly 38 weeks from conception.
Because medical tradition dates pregnancy from the last menstrual period for practical reasons. Conception happens around 2 weeks after LMP in a typical 28-day cycle. So when you're 6 weeks 'pregnant', the embryo is technically only 4 weeks old. This convention is universal in obstetrics.
First trimester covers weeks 1–13. Second trimester covers weeks 14–27. Third trimester covers week 28 up until birth (usually around week 40). Each trimester carries its own developmental focus, symptoms, and standard prenatal testing schedules.
Full term is officially 39 weeks 0 days to 40 weeks 6 days of gestation. Babies born during this window have the best statistical outcomes. Babies born between 37 and 38 weeks are "early term", while those born before 37 weeks are premature.
For IVF, gestational age = transfer date minus embryo age (3 or 5 days) plus 14 days (the standard LMP head start). For a day-5 blastocyst transfer, you are essentially 2 weeks and 5 days pregnant on the exact day of the transfer.
First-trimester ultrasound measures the embryo directly, which is highly accurate. If your periods are irregular or you ovulated late, your LMP math will be wrong. If the ultrasound measurement differs significantly from your LMP estimate, the doctor will redate the pregnancy to match the scan.
Glossary & References
- Gestational Age: The medical measure of a pregnancy's duration, starting from LMP.
- Crown-Rump Length (CRL): The ultrasound measurement from head to bottom used to date pregnancies in the first trimester.
- Quickening: The first maternal perception of fetal movement, typically occurring between 16-22 weeks.
- Viability: The point at which a fetus has a chance of surviving outside the womb, usually considered around 23-24 weeks.
- ACOG: American College of Obstetricians and Gynecologists. (Methods for Estimating the Due Date)
- Cleveland Clinic: Fetal Development Timeline.