Pregnancy Week Calculator
Enter the first day of your last period or your due date – the calculator shows how many weeks and days pregnant you are today, which trimester and month you are in, and how many days are left. It is an arithmetic estimate and does not replace medical advice.
Source: ACOG Committee Opinion No. 700 – Methods for Estimating the Due Date. Updated: .
How it is calculated
How the pregnancy week is calculated
Gestational age is counted from the first day of your last menstrual period (LMP). If you only know your due date, the LMP date is 280 days before it. The calculator counts the days to today (or to the reference date) and splits them into full weeks and days.
How to work out how far along you are
- Under “I know”, choose the first day of your last period or your due date.
- Enter the date; set a different day if you want a date other than today.
- Read the pregnancy week, trimester, month and days left until the due date.
Example: last period on 1 March 2026 – by 28 September 2026, 211 days have passed, i.e. 30 weeks and 1 day (30+1), which is the 31st week of pregnancy.
Example: last period started on 6 May 2026. By 27 September 2026 that is 144 days = 20 full weeks + 4 days, so you are 20 weeks 4 days pregnant – in week 21, second trimester. The due date is Wednesday, 10 February 2027, with 136 days to go; just over half (51%) of the 40 weeks are done.
20 weeks 4 days or week 21?
Both mean the same. “20 weeks 4 days” (written 20+4 in many notes) counts the completed weeks and days; “week 21” names the week you are in.
Trimesters and months
- First trimester: 0 weeks 0 days to 13 weeks 6 days
- Second trimester: 14 weeks 0 days to 27 weeks 6 days
- Third trimester: from 28 weeks 0 days until birth
The calculator counts months of pregnancy in 4-week (28-day) steps, so 40 weeks make 10 “lunar” months – a little more than 9 calendar months.
Important: this is an arithmetic estimate, not a diagnosis, and it does not replace medical advice. After an early ultrasound your clinician may adjust the dates. To work out the due date itself, including a non-28-day cycle, use the due date calculator.
Related calculators: the due date calculator, the ovulation calculator and the date calculator for any span of days.
Frequently asked questions
How many weeks pregnant am I?
Count the days since the first day of your last period and divide by 7: full weeks plus remaining days. 144 days is 20 weeks 4 days, which is week 21 of pregnancy.
How do I work out my pregnancy week from the due date?
Subtract 280 days from the due date to get the calculated start (0 weeks 0 days), then count from there. A due date of 10 February 2027 means a start on 6 May 2026.
What does 12+3 mean?
12 completed weeks and 3 days since the start of your last period. You are in week 13 of pregnancy, still in the first trimester.
When do the second and third trimesters start?
The second trimester starts at 14 weeks 0 days, the third at 28 weeks 0 days.
How many months is a pregnancy?
40 weeks are 10 months of 4 weeks each, or just over 9 calendar months. The calculator shows the month in 4-week steps.
How do I calculate gestational age after IVF or with a longer cycle?
Under “I know” choose the embryo transfer (day 5 or day 3): the due date is then 261 or 263 days after the transfer (ACOG). If you start from your last period, enter your cycle length: with 32 instead of 28 days the due date moves by 4 days. The gestational age in your prenatal records stays the reference.
Sources and legal basis
- ACOG Committee Opinion No. 700 – Methods for Estimating the Due Date
- MedlinePlus – Gestational age
- NHS – Week-by-week guide to pregnancy
As of: · Editorial policy
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