Calculate gestational age today — from your last menstrual period, from an ultrasound measurement, or backward from your due date. The same math your provider uses.
Use the gestational age printed on your scan report, e.g. "8w 3d".
The best due date set by your provider or an early scan.
Prefer a friendlier week-by-week view with a progress bar and countdown? Try the Pregnancy Week Calculator.
Gestational age (GA) is the standard clock of pregnancy: the number of completed weeks and days since the first day of your last menstrual period. It is the number on every scan report, lab slip and prenatal chart — written as "10w 3d" or "10+3". Note what it is not: it is not the age of the embryo. Because conception typically happens around two weeks after the last period begins, fetal (embryonic) age runs about two weeks behind gestational age. A pregnancy at 10 weeks GA carries an embryo roughly 8 weeks old. Medicine settled on the LMP-based clock for a practical reason: the last period is a date most people know, while the moment of conception almost never is.
This calculator finds your GA today three ways. From your last period, it counts the days since (adjusting for your cycle length, since later ovulation in longer cycles shifts real dating later). From an ultrasound, it takes the GA printed on your report and simply adds the days since the scan — the most reliable everyday method once you've had a dating scan. And from a due date, it counts backward, since the due date is by definition the day you'd turn 40 weeks 0 days.
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In the first trimester, embryos grow at a strikingly uniform rate — so measuring one (the crown–rump length) pins down its age with impressive precision. LMP dating, by contrast, depends on an accurately remembered date and on ovulation actually happening around day 14, which long, short and irregular cycles routinely break. That is why the American College of Obstetricians and Gynecologists (ACOG) considers first-trimester ultrasound, up to 13 weeks 6 days, the most accurate method to establish or confirm gestational age.
ACOG's guidance (Committee Opinion 700) even specifies exactly when a scan should override the LMP date. The due date is redated when the discrepancy between ultrasound and LMP dating exceeds:
| Scan performed at | Redate if discrepancy is more than |
|---|---|
| Up to 8w 6d | 5 days |
| 9w 0d – 13w 6d | 7 days |
| 14w 0d – 15w 6d | 7 days |
| 16w 0d – 21w 6d | 10 days |
| 22w 0d – 27w 6d | 14 days |
| 28w 0d and later | 21 days |
One more rule of thumb from the same guidance: once a "best obstetric estimate" is set by an early scan, it generally stays fixed — later ultrasounds measure growth against that date rather than resetting the clock, because dating precision falls as pregnancy advances and babies' sizes naturally diverge.
The calculator does this instantly, but the arithmetic is worth knowing — it is the same arithmetic your provider uses, and it explains why two sources can differ by a day.
Gestational age from an LMP: count the number of days from the first day of your last period up to today, then divide by seven. The whole number is your completed weeks; the remainder is the days. Say your LMP was 4 April 2026 and today is 20 August 2026. That is 138 days. 138 ÷ 7 = 19 with 5 left over, so you are 19 weeks 5 days — written 19w5d. Note the wording: at 19w5d you are in your twentieth week, which is why "how many weeks am I" so often produces two different-sounding answers that are both correct.
Due date from an LMP: add 280 days — 40 weeks — to the same starting date. For a 4 April 2026 LMP that lands on 9 January 2027. The classic shortcut, Naegele's rule, gets you there a different way: subtract three months, add one year, add seven days. In this example that gives 11 January 2027 — two days later than the day count, because the shortcut ignores how many short months the pregnancy happens to span. Both are estimates of the same thing; the 280-day count is the one modern calculators and dating wheels use.
Working backward. To find the date you will reach a given milestone, multiply the weeks by seven and add. 18 weeks 0 days is 126 days after the LMP, so with a 4 April 2026 LMP you hit 18w0d on 8 August 2026. The same logic runs in reverse from a known due date: subtract 280 days to recover the LMP the date was built on.
One caveat the arithmetic hides: LMP dating quietly assumes you ovulated on about day 14 of a 28-day cycle. If your cycles run long, short or irregular, the true conception date shifts and the LMP estimate drifts with it — which is exactly what an early scan is for.
Reports and apps use several abbreviations for the due date, and they all mean the same thing. EDD (estimated date of delivery) is the current standard. EDC (estimated date of confinement) is the older term still printed on plenty of charts. EDB (estimated date of birth) appears in UK and Australian notes. DPP (data provável do parto) is the Portuguese-language equivalent, and FPP (fecha probable de parto) the Spanish one. If your notes say "EDD by LMP" and "EDD by scan", those are the two dating methods above, shown side by side so the discrepancy rule can be applied.
This is the single most common source of confusion in early pregnancy, and the rule is less arbitrary than it looks: the threshold table above is really asking whether the gap is bigger than the scan's own margin of error at that stage. Three worked examples:
Scan at 6w3d by dates, measuring 5w6d. The gap is 4 days. Before 9w0d the threshold is 5 days, so 4 days falls inside the scan's expected error — the LMP date stands and nothing is changed. A four-day "difference" here is not evidence of anything.
Scan at 9w3d by dates, measuring 8w0d. The gap is 10 days, against a 7-day threshold in the 9w0d–13w6d window. The pregnancy is redated to the scan, and the new due date becomes the official one. From that point on, every later number — including this calculator, if you switch it to ultrasound mode — should be built from the scan.
Scan at 25w4d by dates, measuring 23w0d. The gap is 18 days, past the 14-day threshold for that window — but if an early scan already set the due date, this is usually not a dating question at all. Late scans measure size, and size and age come apart as pregnancy advances. A baby measuring well behind established dates in the third trimester is assessed for growth restriction, with follow-up growth scans and Doppler studies, rather than having the calendar moved. The same is true of a 32-week scan that disagrees with the LMP: by then the dating window has long closed.
The underlying principle from ACOG is worth stating plainly — the earliest accurate ultrasound sets the "best obstetric estimate", and once set, it is not revised by later scans. If you know the gestational age assigned on the day of your earliest scan, entering it in ultrasound mode above will reproduce your official dating exactly.
Gestational age is the scheduling backbone of prenatal care. Screening windows are defined by it: the nuchal translucency scan is timed around 11–14 weeks, the anatomy scan around 18–22 weeks, glucose screening around 24–28 weeks, group B strep testing around 36–37 weeks. Lab results are interpreted against it — an hCG level or a fundal height only means something relative to the expected value for that week. And decisions at the edges of pregnancy — managing threatened preterm labor, or discussing induction past 41 weeks — hinge directly on it.
That's also why the birth-timing labels are defined in GA terms. If your result above says "preterm," it simply means birth today would fall before 37 weeks — the label describes the calendar, not a prediction.
| Label | Gestational age at birth |
|---|---|
| Preterm | Before 37w 0d |
| Early term | 37w 0d – 38w 6d |
| Full term | 39w 0d – 40w 6d |
| Late term | 41w 0d – 41w 6d |
| Post-term | 42w 0d and beyond |
The most accurate everyday habit is simple: once you've had a dating scan, use the ultrasound mode of this calculator with the GA from your report. That reproduces your official dating exactly, day for day. Before any scan, LMP mode gives a solid first estimate — just treat it as provisional until your first ultrasound confirms or adjusts it. And if the numbers here ever disagree with what your midwife or doctor tells you, theirs win: they may be working from measurements and history this page can't see.
Gestational age is counted from the first day of your last menstrual period (LMP), not from conception. From the LMP, add the number of days that have passed and express the result as completed weeks plus days — for example, 70 days after your LMP you are 10 weeks 0 days. If dating comes from an ultrasound instead, the scan assigns a gestational age on the day of the scan, and every later date simply adds the days since. This calculator supports both methods, plus dating backward from a known due date.
Gestational age is counted from the last menstrual period, while fetal (or embryonic) age is counted from conception — which typically happens about two weeks later. So a pregnancy at 10 weeks gestational age carries a fetus that is roughly 8 weeks old. Medicine, ultrasound reports and pregnancy apps almost always use gestational age, which is why the two numbers are easy to mix up.
Ultrasound performed in the first trimester (up to 13 weeks 6 days) is the most accurate way to establish gestational age, because early embryos grow at a very predictable rate. LMP dating relies on remembering the date correctly and on ovulating around day 14, which is not true for everyone. When an early ultrasound and the LMP disagree by more than a defined threshold, guidelines from the American College of Obstetricians and Gynecologists recommend using the ultrasound date.
ACOG Committee Opinion 700 sets discrepancy thresholds by scan timing: before 9 weeks 0 days, the due date is redated if ultrasound and LMP differ by more than 5 days; from 9 weeks 0 days to 13 weeks 6 days, by more than 7 days; from 14 to 15 weeks 6 days, more than 7 days; from 16 to 21 weeks 6 days, more than 10 days; from 22 to 27 weeks 6 days, more than 14 days; and at 28 weeks or later, more than 21 days. Once a best due date is set by an early scan, it is generally not changed by later ones.
Count the days from the first day of your last period to today, then divide by seven: the whole number is your completed weeks and the remainder is the days. For example, an LMP of 4 April 2026 is 138 days before 20 August 2026, and 138 divided by 7 is 19 remainder 5 — so 19 weeks 5 days. For the due date, add 280 days to the same LMP. Naegele's rule (subtract three months, add one year, add seven days) is the traditional shortcut and lands within a day or two of the 280-day count.
They are all names for the estimated due date. EDD is estimated date of delivery, the current standard term; EDC (estimated date of confinement) is the older term still found on dating wheels; EDB (estimated date of birth) is common in UK and Australian notes; DPP (data provável do parto) is the Portuguese term and FPP (fecha probable de parto) the Spanish one. When notes show both "EDD by LMP" and "EDD by scan", those are the two dating methods being compared before the redating rule is applied.
It depends on the size of the gap and when the scan was done. A difference smaller than the threshold for that window is within the scan's own margin of error, so the LMP date stands. A larger difference on an early scan means the pregnancy is redated to the ultrasound, and that becomes the official due date. What does not happen is a due date being changed twice: once the earliest accurate scan sets the estimate, later scans measure growth against it rather than resetting it.
By the third trimester, ultrasound is measuring size rather than establishing age — babies of the same gestational age vary widely in size, so dating precision is poor that late. If your due date was already set by an early scan, a later measurement running behind is treated as a growth question, not a calendar question: it is followed up with repeat growth scans and often Doppler studies to check blood flow, and the due date stays where it is. If no early dating scan was ever done, discuss with your provider which estimate they are using.
These are the official labels for when birth happens, by gestational age: preterm is birth before 37 weeks 0 days; early term is 37 weeks 0 days through 38 weeks 6 days; full term is 39 weeks 0 days through 40 weeks 6 days; late term is 41 weeks 0 days through 41 weeks 6 days; and post-term is 42 weeks 0 days or beyond. The distinctions matter because outcomes are best for babies born in the full-term window.