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Gestational Age Calculator: Weeks Pregnant from LMP

What is this tool?

The gestational age calculator tells you how far along a pregnancy is, in weeks and days, from the first day of your last menstrual period, or from an estimated due date. Gestational age is the standard way pregnancy is dated in clinical care: it counts from the last menstrual period, which is about two weeks before conception, so a pregnancy is already counted as two weeks along at conception. Enter your dates below and you get the gestational age, the trimester, and the estimated due date in one result. This page is informational only and is not medical advice; your clinician's dating, especially from ultrasound, takes precedence over any figure here.

How it works

From the last menstrual period, gestational age is the number of days from that date to today, divided into completed weeks and remaining days. The estimated due date is the last menstrual period plus 280 days, which is 40 weeks. From a due date, the calculation runs backwards: gestational age is 280 days minus the days remaining until the due date, which gives the same weeks-and-days figure and is the method to use when a due date has already been set, for example from an ultrasound scan. Trimesters are counted as first trimester up to 13 weeks and 6 days, second trimester from 14 weeks to 27 weeks and 6 days, and third trimester from 28 weeks onwards.

Worked example

Worked example: last menstrual period 1 March 2026. On 23 October 2026 the gestational age is 33 weeks and 5 days, because 236 days have passed since 1 March, and 236 divided by 7 is 33 weeks with 5 days left over. The estimated due date is 1 March plus 280 days, which is 6 December 2026, and the pregnancy is in the third trimester.

When to use it

Gestational age is the number that organises almost everything in pregnancy care, which is why it is asked at every appointment. Screening tests, scans, vaccination timing, leave planning and the decision about when a pregnancy is full term are all tied to gestational age in weeks, not to calendar months, because a week is a precise unit and a month is not. A person who knows their gestational age can follow what is expected at each stage, and a clinician who knows it can time each step correctly.

It is also the number that makes pregnancy comparable between people. Two pregnancies that started in different months can be compared week by week, because gestational age is counted the same way for both. That is why pregnancy tracking, whether in an app, a notebook or a clinic record, is organised by week: the week number is the shared language, and this calculator gives it from the two dates that define it.

• Tracking pregnancy week by week and knowing what stage you are in.
• Working out which trimester you are in for appointments and planning.
• Checking a due date against the last menstrual period or an ultrasound date.
• Planning maternity leave, childcare and practical arrangements around the due date.
• Understanding screening and scan windows, which are given in weeks.
• Comparing notes with a clinician, using the same week-and-day language.

One limit matters: this calculator dates a pregnancy from the dates you enter, and it cannot see what an ultrasound sees. If an ultrasound gives a different due date, especially in the first trimester, clinicians usually prefer the ultrasound date, because it measures the pregnancy directly rather than assuming a standard cycle. If the two dates disagree by more than a few days, ask your clinician which one to use, and use that one consistently afterwards.

Step by step

Using this page takes one starting date and gives the gestational age, the trimester and the due date together.

1. Choose the method: from last menstrual period, or from an estimated due date. Use the due-date method when a date has already been set by a scan or a clinician.
2. Enter the date: the first day of the last menstrual period, or the estimated due date, depending on the method chosen.
3. Enter the reference date, which defaults to 23 October 2026 in the worked example. Change it to today, or to any date you want the gestational age on.
4. Read the headline result: completed weeks and remaining days, for example 33 weeks and 5 days, plus the trimester and the estimated due date.

The weeks-and-days figure is the one to quote, not a rounded number of weeks. Saying 33 weeks when the figure is 33 weeks and 5 days loses almost a week of precision, and at the boundaries between trimesters or before a time-sensitive test, that lost week can matter. The remaining days are part of the result, not decoration, and they should travel with the week figure whenever it is written down.

The trimester shown follows the standard bands: first trimester up to 13 weeks and 6 days, second from 14 weeks to 27 weeks and 6 days, and third from 28 weeks. Different sources draw the lines slightly differently, by a day or two, so if a source you trust uses a different boundary, the week figure is still the same and only the trimester label may differ. Quote the weeks and days, and the trimester will take care of itself.

Common mistakes

The most common mistake is counting from conception instead of from the last menstrual period. Conception happens about two weeks after the last menstrual period in a standard cycle, so counting from conception understates gestational age by about two weeks. That two-week gap is not an error in the calculator; it is the definition of gestational age, and it is why a pregnancy is described as two weeks along at the moment of conception. If your figure looks two weeks too low, check which date you started from.

The second mistake is converting weeks to months by dividing by four. Four weeks are 28 days, but a calendar month is 28 to 31 days, so a pregnancy counted in months by that shortcut drifts from the week count within a month or two. Pregnancy is counted in weeks precisely to avoid that drift, so keep the week figure as the primary number and treat months as a rough translation only.

• Counting from conception instead of from the last menstrual period, understating the age by about two weeks.
• Converting weeks to months by dividing by four, which drifts from calendar months.
• Quoting weeks without the remaining days, losing up to six days of precision.
• Treating the estimated due date as an exact delivery date, when it is an estimate.
• Ignoring an ultrasound date that differs from the last-menstrual-period date.
• Using a due date from one method and a gestational age from another, mixing two different datings.

A further mistake is assuming the due date is a deadline. It is an estimate of 40 weeks, and babies arrive across a range of weeks around it. A pregnancy that passes the due date is not automatically a problem, and a pregnancy that ends before it is not automatically early in a worrying sense; the terms full term, early term and post-term describe ranges, and the clinician watching the pregnancy is the person who judges where it sits. This calculator gives the dates; it does not judge the pregnancy.

More worked examples and notes

The examples below use the same methods as the worked example, and each can be checked by entering the same dates into the tool.

• Last menstrual period 1 March 2026, on 23 October 2026: 33 weeks and 5 days, estimated due date 6 December 2026, third trimester. This is the worked example from the main part of the page.
• From an estimated due date of 6 December 2026, on 23 October 2026: the same 33 weeks and 5 days, because 44 days remain until the due date and 280 minus 44 is 236 days. The two methods agree when the dates are consistent, which is a useful check.
• Last menstrual period 1 January 2026, on 23 October 2026: 42 weeks and 1 day, estimated due date 8 October 2026. The due date has passed, so this pregnancy would be past its estimated date on this reference date, and the person should already be under clinical care for that.
• Last menstrual period 1 September 2026, on 23 October 2026: 7 weeks and 3 days, estimated due date 8 June 2027, first trimester. The due date is 1 September plus 280 days.
• Trimester boundaries: a pregnancy with last menstrual period 1 March 2026 reaches 14 weeks on 7 June 2026, entering the second trimester, and 28 weeks on 13 September 2026, entering the third trimester. Those are the dates the trimester label changes for this pregnancy.

One more note on dating: when a last menstrual period is uncertain, for example after a recent pregnancy, breastfeeding, or irregular cycles, the last-menstrual-period method is less reliable, and ultrasound dating matters more. The calculator will still give a figure from whatever date is entered, but the figure is only as good as the date. If the starting date is uncertain, say so to your clinician, and let the scan set the date.

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Frequently asked questions

Written and checked against the calculator above. Last updated: 23 October 2026. Calculation runs entirely in your browser — no date is sent to a server. This tool gives calendar results only, not medical, legal or financial advice. This tool gives informational results only, not medical advice. Your clinician's dating, especially from ultrasound, takes precedence over any figure here.