Gestational Age Calculator: How Far Along Am I?
Work out today's gestational age in weeks and days from your last period, conception date or due date, with your trimester and how far through the pregnancy you are.
Quick samples
Enter a date to see your gestational age.
Pick a method, add the date (or try a quick sample), and your gestational age in weeks and days, trimester and due date appear here.
This uses Naegele's 280-day (40-week) convention and calendar maths. It is an estimate, and ultrasound dating, especially a first-trimester scan, is more accurate and can shift your gestational age by several days. Use your clinician's dating for any medical decision or scan window.
The formula
Gestational age = days elapsed since the implied LMP, converted to weeks and days (LMP = due date − 280 days)
- LMP — First day of your last menstrual period; day zero of gestational counting.
- 280 days — The standard full-term span of 40 weeks used to convert dates to gestational age.
- Conception — Roughly two weeks after the LMP, so a conception date is treated as LMP plus 14 days.
- Trimester — First under 14 weeks, second to 27, third from 28, by the usual convention.
Gestational milestones by week
| Gestational age | Trimester | Common milestone |
|---|---|---|
| 4–6 weeks | First | Positive test and dating |
| 11–14 weeks | First | Dating and combined screening window |
| 18–22 weeks | Second | Mid-pregnancy anomaly scan |
| 24–28 weeks | Second | Glucose screening |
| 28–40 weeks | Third | Growing to term; birth-window planning |
Windows vary by country and provider. Your booking appointment sets your official dating and the exact schedule, so use that over a general table.
Gestational age is the pregnancy clock that midwives, doctors and scans all speak in, and it is a little counter-intuitive: it counts from the first day of your last period, roughly two weeks before conception actually happened. That is why a pregnancy is described as up to forty weeks even though the baby has only existed for about thirty-eight.
This calculator turns any one anchor date into today's gestational age. Give it a last menstrual period, a conception or transfer date, or a due date, and it converts everything back to that day-zero LMP, then reports how many weeks and days you are on, which trimester that is, how far through the forty weeks you are, and the matching due date.
It also lets you travel in time. Set the check date to any past or future day to see where you would be, which is handy for understanding a scan report from weeks ago or planning around an upcoming appointment. It is calendar maths on the standard forty-week convention, so treat it as an estimate and defer to your clinician's ultrasound dating wherever accuracy matters.
Why we count from the last period
Gestational age starts at the first day of your last menstrual period, not at conception, largely for historical and practical reasons. Most people can name a period start date far more reliably than a conception date, so the medical world standardised on the LMP as a shared day zero everyone can point to.
The cost is that this convention makes a pregnancy look about two weeks longer than the baby's actual age, because ovulation and conception usually happen around day fourteen. Fetal age and gestational age differ by roughly that fortnight, and knowing which one a report uses avoids confusion when numbers seem off.
This calculator keeps you on the gestational convention throughout, so a conception date you happen to know is quietly converted back to an LMP, and everything downstream, weeks, trimester and due date, uses the same anchor a midwife would.
Because of that shared anchor, a twenty-week scan, a twenty-week due-date calculation and a twenty-week period-based figure all mean the same thing, which is exactly what makes tracking appointments and windows straightforward across different providers.
Any of three dates gets you there
You only need one of the three anchors. From a last period date, gestational age is simply the time elapsed since it. From a due date, the tool subtracts 280 days to recover the LMP and counts forward. From a conception or transfer date, it treats that as LMP plus fourteen days.
All three roads lead to the same gestational figure as long as the dates are mutually consistent, which is why an early scan that measures the baby can back-calculate both a due date and an implied LMP and reconcile them into one agreed number.
If your dates disagree, that is normal, and the calculator is showing arithmetic rather than adjudicating. A first-trimester scan is the tie-breaker, so where its dating differs from your remembered dates, trust the scan for appointments and screening windows.
Weeks and days versus a rounded month
Pregnancy is tracked in completed weeks and extra days, written as something like eighteen plus three, because small windows matter for screening and care. A rounded month hides that precision, which is why clinicians ask for weeks and days rather than how many months along you feel you are.
The calculator reports the exact weeks-and-days figure and the percentage through the forty weeks, so you can line yourself up with the many milestones that are defined by a window rather than a month. Those windows are tight on purpose, and an exact count keeps you inside them.
As a guide to feel, the percentage is a friendly way to see progress, but it is the week and day that should go into any conversation about your care, screening or when you might be offered an induction.
Trimesters and the windows that matter
The tool labels your trimester using the standard split: the first runs to just under fourteen weeks, the second to week twenty-seven, and the third from twenty-eight onward. Trimesters are how people describe symptoms and energy, even though care actually keys off the precise weeks.
Several key appointments are tied to narrow date ranges rather than trimesters, including dating scans, the combined screening bloods and ultrasound, and the mid-pregnancy anomaly scan, all of which specify a minimum and maximum gestational age for a valid result.
That is the practical reason to keep an exact count: knowing you are, say, twelve and four today tells you whether you are inside a screening window this week or need to schedule for next week, in a way that a vague second-trimester label never could.
A worked example
Someone who does not know their period start but has a due date enters the due date and selects the due-date method. The calculator subtracts 280 days to find the implied LMP, counts the days from it to today, and returns a gestational age of about thirty weeks with a trimester of third.
It also shows the implied LMP and the percentage through the forty weeks, so the same person can see they are roughly three-quarters of the way, with around seventy days to the due date, and can sanity-check that against how they feel.
Had they known a conception date instead, choosing that method would treat it as LMP plus fourteen days and land on the same thirty-week figure, which demonstrates that the anchor you enter does not change the answer as long as the dates agree.
Key terms
- Gestational age
- How far a pregnancy has progressed, counted in weeks and days from the first day of the last period.
- LMP
- Last menstrual period; the day-zero anchor for gestational counting.
- Fetal age
- The baby's actual age since conception, about two weeks less than gestational age.
- Dating scan
- An early ultrasound that measures the baby to confirm or correct gestational age.
Pro tips
- Give any provider weeks and days, not months, so screening windows are right.
- If dates are uncertain, a dating scan settles gestational age more reliably than memory.
- Use the as-of date to decode an old scan report or plan an upcoming appointment.
- Trust a first-trimester scan over calculated dates when the two disagree.
Common mistakes
- Confusing gestational age with the baby's actual age since conception.
- Rounding to months and missing a time-limited screening window.
- Assuming a remembered due date beats an early scan when they differ.
- Entering a conception date but leaving the LMP method selected.
Gestational age — your questions
Why is my gestational age two weeks more than the baby's age?
Gestational age counts from the first day of your last period, which is about two weeks before ovulation and conception, so it is always roughly a fortnight ahead of the baby's actual age since conception. Both conventions are valid; medical care simply standardises on the last-period anchor because more people can recall it reliably than the exact day of conception.
Which date should I enter?
Any one of three works: the first day of your last period, a conception or embryo-transfer date, or a due date. The calculator converts each to the same day-zero anchor and returns an identical gestational age as long as the dates are consistent. Use whichever you know most confidently, and if you have a scan-confirmed due date, that is usually the best single number to trust.
Is a scan or my calculated date more accurate?
An early ultrasound, particularly a first-trimester dating scan, is generally the most reliable estimate of gestational age, because it measures the baby directly rather than relying on remembered dates or cycle regularity. If a scan gives a different figure from your period-based dates, your provider usually adopts the scan date, so use it for appointments and screening windows.
Can I find how far along I was on a past date?
Yes. The as-of field defaults to today but you can set it to any date to see the gestational age on that day. This is handy for reading an older scan report, checking whether you were inside a screening window at the time, or planning around a future appointment, all from the same anchor date you enter.
How accurate is this calculator?
It is precise at the arithmetic, applying the standard 280-day, forty-week convention exactly, but pregnancy itself is not that neat. Estimated due dates are accurate to within a few days for most people, and only a minority of babies arrive on the predicted date. Treat the weeks-and-days figure as a very good estimate and rely on your clinician's dating for anything time-critical.