Gestational Age Calculator

Gestational Age Calculator. Calculate gestational age using either your last menstrual period (LMP) or an ultrasound date. Enter the current date and either your LMP date or your known due date, and get back gestational age in weeks and days, your estimated due date (EDD), and your current trimester. Also try the Period Calculator.

Calculate By *

Choose whether to calculate from your LMP or from a known due date.

Today's date or the date for which you want gestational age calculated.

The first day of your last menstrual period. Used to calculate gestational age and due date.

Your estimated due date if already known (e.g. from an ultrasound).

Results

Gestational Age (Weeks)

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Gestational Age

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Estimated Due Date (EDD)

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Current Trimester

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Days Until Due Date

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Approximate Conception Date

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What is gestational age and how is it measured?

Gestational age is the age of a pregnancy measured in weeks and days, starting from the first day of the mother's last menstrual period (LMP). It is the standard measure used by healthcare providers and typically runs about 2 weeks ahead of the actual age of the embryo or fetus. See also our Luteal Phase Length — Luteal Phase.

Why is gestational age counted from the last menstrual period?

Counting from the LMP is a long-standing medical convention because the exact date of ovulation or fertilization is usually unknown. Since the LMP is a known, documented date, it provides a reliable and consistent reference point that all healthcare providers can use.

What is the difference between gestational age and fetal age?

Gestational age counts from the LMP, while fetal age (also called embryonic age or conception age) counts from the actual day of fertilization, which is typically about 2 weeks after the LMP. Gestational age will therefore always be approximately 2 weeks greater than fetal age.

How is the estimated due date (EDD) calculated from the LMP?

The most common method is Naegele's Rule: add 280 days (40 weeks) to the first day of the last menstrual period. This assumes a 28-day menstrual cycle with ovulation on day 14. The EDD represents the end of a full-term pregnancy.

How accurate is gestational age calculated from LMP?

LMP-based dating is a reasonable estimate but can be inaccurate if your cycles are irregular or if you are unsure of your LMP date. First-trimester ultrasound (before 14 weeks) is considered the most accurate method for dating a pregnancy and is often used to confirm or adjust the LMP-based EDD.

What are the three trimesters of pregnancy?

The first trimester spans weeks 1–13, the second trimester covers weeks 14–26, and the third trimester runs from week 27 through birth (typically week 40). Each trimester marks important developmental milestones for the baby.

Can I calculate gestational age if I only know my due date?

Yes. If you know your due date (from an ultrasound, for example), you can work backwards: your LMP would be approximately 280 days before that date. This calculator supports due-date-based entry so you can find gestational age even if you are unsure of your LMP.

Is this calculator a substitute for medical advice?

No. This tool is for informational and educational purposes only. Always consult your obstetrician or midwife for clinical pregnancy dating and medical guidance. Accurate dating is important for pregnancy care and should be confirmed by a healthcare professional. You might also find our calculate NT Percentile, Expected NT for CRL & NT / Expected NT Ratio — Nuchal Translucency useful.