Gestational Age Calculator
Weeks pregnant, trimester and due date from your last period.
Gestational age
0 weeks 0 days
First trimester
Trimester
First trimester
Estimated due date
June 24, 2027
Days remaining
279
AI Breakdown & Smart Takeaway
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How the Gestational Age Calculator works
The Gestational Age Calculator determines how many weeks pregnant you are, which trimester you're in, and your estimated due date — all from the first day of your last menstrual period (LMP). It's designed for anyone in early pregnancy who wants a quick, medically grounded starting point before their first prenatal appointment.
Gestational age is calculated from the first day of your last menstrual period, not from the date of conception. This convention, established by obstetric medicine, adds roughly two weeks to what most people intuitively think of as 'time pregnant,' because ovulation and fertilization typically occur around day 14 of a 28-day cycle. So when a doctor says you are 6 weeks pregnant, the embryo itself is only about 4 weeks old. The calculator uses your LMP date and today's date to compute the difference in days, then converts that figure into completed weeks and days.
The standard formula for estimating your due date is Naegele's Rule: take the first day of your LMP, subtract 3 months, and add 7 days (then adjust the year forward if necessary). This produces an estimated due date (EDD) at 280 days, or exactly 40 weeks, from the LMP. While this rule assumes a perfect 28-day cycle with ovulation on day 14, it remains the global clinical standard and is what most ultrasound dating is ultimately compared against in the first trimester.
Your current trimester is determined by where your gestational age falls within three defined windows. The first trimester spans weeks 1 through 13, covering the period of highest miscarriage risk and critical organ formation. The second trimester runs from week 14 through week 27, generally considered the most comfortable phase, during which anatomy scans are performed. The third trimester begins at week 28 and continues until delivery, encompassing rapid fetal weight gain and the preparations for birth. Knowing your trimester helps you anticipate which prenatal screenings, appointments, and developmental milestones apply to you right now.
A common mistake is entering the wrong date — specifically, using an ovulation date, a positive pregnancy test date, or a conception estimate instead of the true LMP. These errors can shift the gestational age calculation by one to two weeks, leading to confusion when compared against a clinician's dating. If your cycles are significantly longer or shorter than 28 days, or if an early ultrasound (performed between 8 and 13 weeks) gives a different date, healthcare providers will typically adjust the EDD to match the ultrasound measurement, which is the most accurate dating method available in the first trimester.
Formula
Weeks since first day of last menstrual period (Naegele's rule)
Pro tips
- Always enter the first day of your period — not the last day and not when you think you ovulated. Even a 2-day error in your LMP can shift your gestational age and EDD noticeably.
- If your cycles are consistently longer than 28 days (e.g., 35 days), ovulation likely occurred around day 21 rather than day 14, meaning your actual conception was later; flag this for your provider, as your ultrasound EDD may differ from the LMP-based one.
- Screenshot or note your calculated EDD before your first prenatal visit — if your provider's dating matches within 5–7 days, the LMP date will typically be kept; a larger discrepancy usually means the ultrasound measurement takes precedence.
- Use the trimester information to look up which prenatal tests are time-sensitive: first-trimester screening (nuchal translucency + blood test) must be done between weeks 11 and 13+6, and the anatomy scan is scheduled around weeks 18–20.
- Remember that only about 5% of babies are born on their exact EDD — a normal delivery can occur anywhere from 37 to 42 weeks, so treat the due date as the center of a window, not a deadline.
Key terms
- Last Menstrual Period (LMP)
- — The first day of the most recent menstrual period before pregnancy, used as the universal starting point for calculating gestational age.
- Gestational Age
- — The age of a pregnancy measured in weeks and days from the first day of the LMP, typically running 2 weeks ahead of the embryo's actual age.
- Estimated Due Date (EDD)
- — The projected date of delivery at 40 weeks of gestation, calculated using Naegele's Rule or confirmed by first-trimester ultrasound.
- Naegele's Rule
- — The standard obstetric formula for estimating due date: subtract 3 months and add 7 days to the first day of the LMP.
- Trimester
- — One of three roughly equal phases of pregnancy — first (weeks 1–13), second (weeks 14–27), and third (weeks 28–40) — each associated with distinct fetal development milestones and prenatal care tasks.
- Crown-Rump Length (CRL)
- — An ultrasound measurement of the embryo from head to bottom used in the first trimester to confirm or adjust gestational age when LMP dating is uncertain.