Gestational Age Calculator

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Enter the first day of your last menstrual period to calculate your gestational age, estimated due date, and pregnancy milestones.

This tool provides estimates based on Naegele's rule and is for informational purposes only. It is not a substitute for professional medical advice. Always consult your healthcare provider for accurate pregnancy dating and medical guidance.
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    About This Tool

    The Gestational Age Calculator estimates how far along a pregnancy is using Naegele's rule, the standard method recommended by the American College of Obstetricians and Gynecologists (ACOG). It calculates gestational age in weeks and days, the estimated due date, current trimester, and term classification — all directly in your browser with no data sent anywhere.

    How to Use

    1. Enter the first day of your last menstrual period.
    2. Results appear automatically with your gestational age and due date.
    3. Review your trimester, term classification, and pregnancy milestones.

    Methodology

    This calculator uses Naegele's rule, which estimates the due date by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). This method assumes a 28-day menstrual cycle with ovulation on day 14, and has been the standard in obstetric practice since the early 19th century. Trimester boundaries follow ACOG definitions: the first trimester spans weeks 1–13, the second spans weeks 14–27, and the third spans weeks 28–40. Term classifications follow the 2013 ACOG-SMFM joint recommendation. Research shows that only about 4% of births occur on the exact estimated due date, with most occurring within a two-week window around it.

    Sources: ACOG Committee Opinion #700 · Naegele's Rule

    Understanding Your Results

    Your gestational age is shown in the standard weeks-and-days format used by healthcare providers (for example, "24 weeks and 3 days" or "24w3d"). The trimester indicates which phase of pregnancy you are in, and the term classification tells you whether the pregnancy is preterm, early term, full term, late term, or post-term per the 2013 ACOG definitions. Fetal age is approximately two weeks less than gestational age, reflecting the time between your LMP and likely conception. The progress bar and milestone dates help you visualize the pregnancy timeline from beginning to delivery window.

    Sources: ACOG Committee Opinion #579

    Sources

    Practical Examples

    If your last menstrual period started on January 1, 2025, your estimated due date would be October 8, 2025 (January 1 + 280 days). On June 15, 2025, you would be 23 weeks and 4 days into your pregnancy — in the second trimester, with a fetal age of approximately 21 weeks and 4 days. By September 10, 2025, you would be 36 weeks and 1 day — still preterm. Full term begins at 39 weeks 0 days, which would be September 24, 2025. Your delivery window spans from early term (37 weeks, September 10) through post-term (42 weeks, October 22).

    Pregnancy Tracking Tips

    • Record the first day of your last menstrual period as soon as possible — this date is the foundation for all pregnancy dating calculations. • Schedule your first prenatal visit early, ideally between weeks 8 and 12. An early ultrasound can confirm or refine your due date. • Remember that your due date is an estimate, not a deadline. Most babies arrive within two weeks before or after the estimated date. • Each trimester brings different needs: the first focuses on early development, the second on growth and movement, and the third on preparing for delivery. • Keep track of your milestones and share them with your healthcare provider at each visit to stay informed about your pregnancy progress.

    All calculations are performed locally in your browser. No data is sent to any server.

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    Frequently Asked Questions
    What is gestational age?
    Gestational age is the number of weeks and days that have passed since the first day of your last menstrual period (LMP). It is the standard way healthcare providers measure pregnancy progress. A full-term pregnancy is considered 40 weeks of gestational age, even though conception typically occurs about two weeks after the LMP. This dating convention is used worldwide by obstetricians and midwives.
    What is Naegele's rule and how does it work?
    Naegele's rule is the most widely used method to estimate a due date. It was first described by German obstetrician Franz Karl Naegele in 1812. The rule works by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a 28-day menstrual cycle with ovulation occurring on day 14. For example, if your LMP was January 1, adding 280 days gives an estimated due date of October 8. While simple and practical, the rule is most accurate for women with regular 28-day cycles.
    How accurate is this gestational age calculator?
    This calculator uses Naegele's rule, which assumes a standard 28-day menstrual cycle. For women with regular cycles close to 28 days, the estimated due date is typically accurate within a window of about two weeks. However, only around 4% of babies are born on their exact due date. Accuracy may vary if your cycles are shorter or longer than 28 days, or if they are irregular. In clinical practice, healthcare providers may adjust the due date based on early ultrasound measurements, which can be more precise for dating, especially during the first trimester.
    What are the three trimesters of pregnancy?
    Pregnancy is divided into three trimesters, each lasting about 13 weeks. The first trimester (weeks 1–13) is when major organ systems begin to form. The second trimester (weeks 14–27) is often considered the most comfortable period, during which fetal movement becomes noticeable. The third trimester (weeks 28–40) is characterized by rapid fetal growth and preparation for birth. Each trimester brings different developmental milestones and physical changes for the mother. Your healthcare provider will schedule specific tests and checkups based on which trimester you are in.
    What does "full term" mean in pregnancy?
    In 2013, the American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine redefined pregnancy term classifications to better reflect outcomes. The categories are: Early Term (37 weeks 0 days through 38 weeks 6 days), Full Term (39 weeks 0 days through 40 weeks 6 days), Late Term (41 weeks 0 days through 41 weeks 6 days), and Post-Term (42 weeks 0 days and beyond). Babies born during the full-term window (39–40 weeks) generally have the best health outcomes. This is why elective deliveries before 39 weeks are discouraged unless medically indicated.
    What is the difference between gestational age and fetal age?
    Gestational age counts from the first day of your last menstrual period (LMP), while fetal age (also called embryonic age or conceptional age) counts from the estimated date of conception. Because ovulation and conception typically occur about two weeks after the start of the LMP, fetal age is approximately two weeks less than gestational age. For example, at 12 weeks gestational age, the fetal age is approximately 10 weeks. Healthcare providers almost always use gestational age as the standard measurement because the LMP date is usually known, whereas the exact date of conception is often uncertain.
    Is ultrasound dating more accurate than using the LMP?
    A first-trimester ultrasound (before 14 weeks) is generally considered more accurate for dating than LMP-based calculation, with an accuracy of plus or minus 5 to 7 days. The ACOG recommends that if the ultrasound date differs from the LMP-based date by more than 7 days in the first trimester, the ultrasound date should be used to determine the due date. Later ultrasounds become progressively less accurate for dating because fetal growth rates vary more as pregnancy advances. This is why early ultrasound dating is preferred when available. However, LMP-based calculation remains a reliable starting point that can be refined with ultrasound findings.
    Is my data private when using this calculator?
    Yes, your data is completely private. All calculations are performed directly in your browser — no information is sent to any server. Your LMP date and pregnancy details are never stored, transmitted, or shared. You can use the calculator with full confidence that your health information remains entirely on your device.