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.
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.
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.
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.
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.
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.
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.
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.
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.