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Gestational Age Calculator
How many weeks pregnant are you? Gestational age in weeks and days on any date, from your last period, due date or conception date.
Gestational age is how far along a pregnancy is, written in weeks and days — for example 24+3 means 24 weeks and 3 days. Midwives and doctors use it to time scans, tests and check-ups, and to judge whether a baby is growing as expected. It is counted from the first day of the last menstrual period, about two weeks before conception, so you are already considered two weeks pregnant at conception.
This calculator works out gestational age on any date — today, the day of an appointment, or a date in the past. Start from the first day of your last period, your due date from a scan, or a known conception date. You also see the trimester, due date, days to go and the dates of key milestones. Your dates are not stored or sent, and are not added to the page address.
How to use Gestational Age Calculator
- Choose what you know: last period, due date or conception date.
- Enter that date.
- Choose the date to calculate for (today by default).
- Read the gestational age, trimester, due date and milestones.
Features
- Weeks and days of pregnancy on any date.
- From the last period, a due date or a conception date.
- Trimester and week of pregnancy.
- Due date and days to go.
- Dates of key milestones.
- Private: dates are not stored, sent or added to the address.
How gestational age is counted
Pregnancy dating starts from the first day of the last menstrual period, because that date is usually known while the day of conception is not. A full-term pregnancy is 40 weeks, or 280 days, from that date. If you know the due date — for example from a dating scan — the calculator counts back 280 days to find the start. If you know the conception date, it counts back 14 days, since ovulation is assumed on day 14 of a 28-day cycle. Gestational age is then the number of whole weeks and leftover days between that start and the date you choose.
Trimesters and milestones
Pregnancy is divided into three trimesters. These milestones are shown with their dates for your pregnancy.
| Weeks | Stage |
|---|---|
| 0–13 | First trimester |
| 12 | Dating scan is usually done around now |
| 14–27 | Second trimester |
| 20 | Halfway; anatomy scan around this time |
| 28 onwards | Third trimester |
| 37 | Early term |
| 39 | Full term |
| 40 | Due date |
Gestational age and week of pregnancy
People often mix up 'weeks pregnant' and 'week of pregnancy'. At 10 weeks and 3 days, ten weeks are complete and you are in your eleventh week, just as a baby who is 10 months old is in their eleventh month. The calculator shows both. Embryonic or fetal age — the time since conception — is about two weeks less than gestational age, which is why it appears in some ultrasound reports and IVF clinics.
Accuracy
Dating from the last period assumes a regular 28-day cycle with ovulation around day 14. If your cycles are longer, shorter or irregular, or you are unsure of the date, an early ultrasound gives a more accurate gestational age, and your care team will usually use the scan date from then on. Only about one baby in twenty is born on the due date itself; most arrive between 37 and 42 weeks. This tool is for information only — always follow the dates and advice of your midwife or doctor. To work out a due date in more ways, including IVF transfer dates, use the Due Date Calculator.
Frequently asked questions
How many weeks pregnant am I?
Enter the first day of your last period (or your due date) and the calculator shows the weeks and days.
When does pregnancy start counting?
From the first day of your last period, about two weeks before conception.
How long is a full-term pregnancy?
40 weeks, or 280 days, from the first day of the last period.
Can I calculate gestational age for a past or future date?
Yes. Change the 'On this date' field.
My scan gave a different date — which is right?
Use the scan date; early ultrasound dating is more accurate than the last period.
What does 24+3 mean?
24 completed weeks and 3 days of pregnancy, counted from the first day of the last period.
Is my information stored?
No. Dates are calculated in your browser and are not saved, sent or added to the page address.
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Pregnancy Weight Gain CalculatorRecommended pregnancy weight gain from your pre-pregnancy BMI, the healthy range for your current week, and how your gain compares.
Ovulation CalculatorFind your most fertile days, estimated ovulation date and next period for the next six cycles, with a calendar of your current cycle.
Period CalculatorPredict your next periods: the next six start and end dates from your last period and cycle length, with a window for irregular cycles.
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