Gestational age is how far along a pregnancy is, counted in completed weeks and days from the first day of the last menstrual period. It is written as "32w 4d" and it is the unit every part of maternity care runs on — screening windows, growth charts, viability thresholds, and the timing of delivery are all defined against it, never against the embryo's actual age.
- What it measures
- Completed weeks and days since the LMP
- Written as
- Weeks + days, e.g. 32w 4d
- Full pregnancy
- 40 weeks 0 days from the LMP
- Embryonic age
- Always 2 weeks less than gestational age
- Trimesters
- 0–13w6d, 14w0d–27w6d, 28w0d onwards
Formula
LMP-based dating (Naegele's 280-day convention)Gestational age (days) = today − LMP Gestational age (days) = 280 − (due date − today) Weeks = ⌊days ÷ 7⌋, days = remainder
- LMP
- — First day of the last menstrual period
- 280
- — Days from the LMP to the due date — 40 weeks
- ⌊ ⌋
- — Round down to whole weeks; the remainder is the days
Gestational age is expressed in completed units: 13 weeks and 6 days is written 13w6d and means 97 days have elapsed, not that you are in your 13th week. The convention matters because screening windows are specified this way — "11w0d to 13w6d" is a precise 21-day span, not an approximation.
Gestational Age Calculator examples
| Input | Result | Category |
|---|---|---|
| LMP Jan 15, 2026 · on Mar 12, 2026 | 8w | First trimester |
| LMP Jan 15, 2026 · on Jun 1, 2026 | 19w 4d | Second trimester |
| LMP Jan 15, 2026 · on Sep 20, 2026 | 35w 3d | Third trimester |
| Due Oct 22, 2026 · on Mar 12, 2026 | 8w | First trimester |
| Due Oct 22, 2026 · on Aug 1, 2026 | 28w 2d | Third trimester |
| Due Oct 22, 2026 · on Oct 20, 2026 | 39w 5d | Third trimester |
How many weeks pregnant am I?
Count the days from the first day of your last period to today and divide by seven. The whole number is your weeks; the remainder is your days. Someone whose last period began on 19 June and who is checking on 14 August is 56 days along — exactly 8 weeks 0 days.
If you know your due date but not your last period, subtract the days remaining from 280 instead. A due date 224 days away also gives 56 days, or 8w0d. Both routes are the same calculation from opposite ends.
Why is gestational age two weeks more than the embryo's age?
Because the clock starts before conception. Obstetric dating counts from the first day of the last menstrual period, which precedes ovulation by roughly two weeks. At the moment of conception you are already counted as two weeks pregnant, and the offset persists for the rest of the pregnancy.
The convention survives because it is practical, not because it is biologically tidy. The date of a period is knowable; the date of conception usually is not.
What are the trimesters?
Trimesters divide the pregnancy into three roughly equal stretches, each with its own characteristic risks and milestones. The boundaries are conventions rather than biological switches, and you may see slightly different cut-points elsewhere.
| Trimester | Gestational age | What defines it |
|---|---|---|
| First | 0w0d – 13w6d | Organ formation; highest miscarriage risk |
| Second | 14w0d – 27w6d | Growth and movement; anatomy scan |
| Third | 28w0d – birth | Rapid growth; preparation for delivery |
The first trimester is the one where the boundary varies most between sources — some place it at 12w6d rather than 13w6d. This calculator follows the more common convention of ending it at 13w6d, which aligns with the end of the first-trimester screening window.
What are the key gestational age milestones?
Most of the fixed points in maternity care are specified in weeks and days rather than in months, and several of them are genuinely narrow windows that cannot be caught up later.
| Gestational age | Milestone |
|---|---|
| 4w0d | Missed period; home tests reliably positive |
| 6w0d | Cardiac activity usually visible on ultrasound |
| 8w0d–10w0d | First prenatal visit and dating scan |
| 10w0d | Earliest date for cell-free DNA screening |
| 11w0d–13w6d | First-trimester combined screening |
| 18w0d–22w0d | Anatomy (mid-pregnancy) ultrasound |
| 24w0d | Generally accepted threshold of viability |
| 24w0d–28w0d | Gestational diabetes screening |
| 27w0d–36w0d | Tdap vaccination |
| 36w0d–37w6d | Group B streptococcus swab |
| 39w0d | Full term begins |
The first-trimester screening window is the least forgiving of these: it closes at 13w6d and cannot be reopened, which is one of the strongest arguments for getting an early dating scan rather than relying on a remembered period date.
How accurate is gestational age from a last period?
Less accurate than most people assume. It rests on two assumptions — that the period date is remembered correctly, and that ovulation happened on day 14 — and both are often wrong. Studies of hormone-tracked cycles find ovulation day varies by several days even among women with regular cycles.
A crown-rump length measured before 14 weeks dates a pregnancy to within about five days. ACOG advises redating when an early ultrasound and the LMP disagree by more than the scan's own margin — more than 5 days before 9 weeks, or more than 7 days from 9 to 16 weeks. After 22 weeks ultrasound dating becomes unreliable and the original date is usually kept.
Frequently asked questions
How do I calculate how many weeks pregnant I am?
Count the days from the first day of your last period to today, then divide by seven. The whole number is your weeks and the remainder is your days. If you know your due date instead, subtract the days remaining from 280 and divide that.
What does 32w 4d mean?
Thirty-two completed weeks and four days since the first day of your last period, or 228 days in total. Obstetric notation always uses completed units, so 32w4d means you have finished 32 weeks and are four days into your 33rd week.
Is gestational age the same as fetal age?
No. Gestational age counts from the first day of your last period; fetal or embryonic age counts from conception, about two weeks later. A pregnancy at 20 weeks gestational age contains an 18-week-old fetus. Clinical care, scan reports, and screening windows all use gestational age.
When does the second trimester start?
At 14 weeks 0 days, immediately after 13 weeks 6 days. It runs to 27 weeks 6 days, with the third trimester beginning at 28 weeks 0 days. Some sources place the first-to-second boundary a week earlier, at 13 weeks 0 days.
What is the earliest gestational age a baby can survive?
Around 22 to 24 weeks with intensive neonatal care, though survival at the lower end is uncommon and often accompanied by long-term complications. Twenty-four weeks is the threshold most commonly used clinically, and outcomes improve substantially with each additional week.
Why did my due date change after my scan?
Because an early ultrasound dates a pregnancy more precisely than a remembered period does. If the two disagree by more than the scan's margin of error — about five days before nine weeks — ACOG advises adopting the scan date. This is routine, not a warning sign.
Sources
- Methods for Estimating the Due Date (Committee Opinion 700) — American College of Obstetricians and Gynecologists, 2017
- Definition of Term Pregnancy (Committee Opinion 579) — American College of Obstetricians and Gynecologists, 2013
- How Your Fetus Grows During Pregnancy — American College of Obstetricians and Gynecologists
- Fetal development — MedlinePlus, U.S. National Library of Medicine