Gestational Age Calculator
Gestational age is counted from the first day of your last menstrual period (LMP), not from the date of conception. This method is used universally by healthcare providers because the exact date of ovulation and conception is usually unknown, while the LMP date is typically well-remembered. A full-term pregnancy is 40 weeks from LMP (280 days). This calculator takes your LMP date and today's date to calculate your current gestational age in weeks and days, your estimated due date (EDD) using Naegele's rule, your current trimester, and how many days remain until the EDD. It also shows key developmental milestones at each gestational age.
Gestational age formula (Naegele's rule)
Gestational Age = Today's Date - LMP Date (in days)
Gestational Age (weeks) = Days / 7 (integer weeks and remainder days)
EDD = LMP + 280 days
Gestational age: frequently asked questions
What is gestational age?
Gestational age is the number of weeks and days of pregnancy, counted from the first day of the last menstrual period (LMP). It is the standard method used by obstetricians. Full-term pregnancy is 40 weeks (280 days) from LMP, even though conception typically occurs around day 14 of the menstrual cycle.
How is the estimated due date calculated?
Naegele's rule: add 280 days (40 weeks) to the first day of the LMP. Alternatively, add 9 months and 7 days to the LMP date. Ultrasound dating is considered more accurate, especially when performed in the first trimester. Most babies are born between 38-42 weeks.
What are the trimesters?
First trimester: weeks 1-13. Second trimester: weeks 14-27. Third trimester: weeks 28-40+. Premature birth is before 37 weeks. Early term is 37-38 weeks; full term is 39-40 weeks; late term is 41 weeks; post-term is 42 weeks or beyond.
What if my cycle is not 28 days?
The standard calculation assumes a 28-day cycle with ovulation on day 14. If your cycle is shorter or longer, the calculated EDD may be off. For example, a 35-day cycle means ovulation on approximately day 21, so EDD would be 7 days later than the standard calculation. Ultrasound provides the most accurate dating.
When should I have my first prenatal visit?
The American College of Obstetricians and Gynecologists (ACOG) recommends the first prenatal visit between 6-8 weeks of pregnancy (6 weeks of gestational age). Early ultrasound confirms dates, viability, and number of embryos. Contact your healthcare provider as soon as you have a positive pregnancy test.
Official sources
- American College of Obstetricians and Gynecologists: Methods for Estimating the Due Date.
- CDC: Pregnancy.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.