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Gestational Age Calculator

Wondering how many weeks pregnant you are? Enter the first day of your last menstrual period and this Gestational Age Calculator shows your exact age of pregnancy in weeks and days, the trimester you’re in, and your estimated due date. Gestational age is counted from your last period — the obstetric standard — so it’s the same number your provider and pregnancy app use. It’s a quick planning snapshot you can check any day, no appointment or sign-up needed, and nothing you type ever leaves your device.

Use the date your most recent period started — gestational age is measured from this day, not from conception.

Pick the first day of your last menstrual period to see how many weeks pregnant you are and your estimated due date.

How gestational age is calculated

Gestational age is the age of a pregnancy measured from the first day of your last menstrual period (LMP). The math is simple counting: days since LMP ÷ 7 = weeks, with the remainder as days. If 59 days have passed since your last period started, that’s 8 full weeks (56 days) plus 3 leftover days — so you’re 8 weeks and 3 days, usually written 8w3d.

It feels odd at first, but pregnancy is counted from LMP rather than from conception. Ovulation and conception happen roughly two weeks after the start of your period, so on the day you conceive you’re already counted as about 2 weeks pregnant. That’s why a full-term pregnancy is 40 weeks even though the baby has only been developing for around 38.

Trimesters and your due date

Your gestational age also tells you which trimester you’re in and when your baby is due:

  • First trimester (weeks 0–13): organs form and early symptoms like nausea and fatigue tend to peak.
  • Second trimester (weeks 14–27): often the most comfortable stretch, with the anatomy ultrasound around week 20.
  • Third trimester (weeks 28 onward): fastest growth, more frequent check-ups, and prep for birth.
  • Estimated due date: LMP + 280 days (40 weeks). Most babies arrive between 38 and 42 weeks.

Want a week-by-week look at what’s happening? Our pregnancy week-by-week guide walks through size, development, and symptoms for each week.

Why your gestational age might be adjusted

LMP-based dating assumes a textbook 28-day cycle with ovulation on day 14. Real cycles vary, so your number can shift for a few reasons:

  • An early dating ultrasound (before week 14) measures the baby’s size directly and usually takes priority over the LMP date.
  • Your typical cycle is longer or shorter than 28 days, moving your ovulation day earlier or later.
  • You weren’t certain of the exact first day of your last period, or spotting was mistaken for a period.
  • You conceived through IVF, where the embryo transfer date gives a more exact starting point than LMP.

When to check in with your provider

This calculator is a planning tool, not a clinical assessment. Your gestational age and due date should always be confirmed at a prenatal visit. Reach out to your midwife, doctor, or maternity service if:

  • You have a positive test but haven’t booked your first prenatal appointment yet — early care matters.
  • You’re unsure of your LMP date or your cycles are very irregular, so the estimate here may be off.
  • You have bleeding, cramping, severe pain, persistent vomiting, fever, or any symptom that worries you at any stage of pregnancy.
  • Your provider’s ultrasound date differs from this estimate — always go with the date your care team gives you.

Frequently asked questions

What is gestational age?
Gestational age is how far along a pregnancy is, counted from the first day of the last menstrual period (LMP) — not from conception. It is written as weeks and days, like 8 weeks 3 days, often shortened to 8w3d. Because counting starts about two weeks before conception actually happens, gestational age is roughly two weeks more than the embryo's true age.
How do you calculate gestational age from the last period?
Count the number of days from the first day of your last menstrual period up to today, then divide by seven. The whole number is your completed weeks and the remainder is the extra days. If 59 days have passed, that is 8 full weeks (56 days) plus 3 days, so you are 8 weeks 3 days. This calculator does that math for you instantly.
Which trimester am I in?
Trimesters are grouped by completed weeks: the first trimester runs from week 0 through week 13, the second from week 14 through week 27, and the third from week 28 until birth. This tool labels your current trimester automatically from your gestational age.
How is the due date worked out?
The estimated due date (EDD) is the first day of your last period plus 280 days, which is 40 weeks. This is the standard obstetric formula, known as Naegele's rule. Only about 1 in 20 babies arrives on the exact due date — most are born within two weeks either side, between 38 and 42 weeks.
Why might my ultrasound date differ from this one?
LMP-based dating assumes a regular 28-day cycle with ovulation on day 14. If your cycles are longer, shorter, or irregular, the date can be off by a week or more. An early ultrasound (before 14 weeks) measures the baby's actual size, so providers usually treat that date as more accurate and may adjust your due date accordingly.

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— The Period Tools Team