How Many Weeks Pregnant Am I? Calculator

Once you know you are pregnant, one of the first things most people want to know is exactly how far along they are, right now, today. This calculator answers that directly: enter your last period, conception date or IVF transfer date, and it works out your current week and day of pregnancy, your trimester, and your next NHS milestone, rather than just giving you a single due date.

How to use the pregnancy weeks calculator

Choose how you want to calculate from: your last menstrual period, your conception date, or an IVF transfer date, since each uses a slightly different dating method. If you choose last period, also enter your average cycle length, which defaults to 28 days but can be adjusted for shorter or longer cycles. The calculator then shows how many weeks and days pregnant you are as of today, your current trimester, how many days remain until your estimated due date, and the next milestone appointment coming up in NHS antenatal care.

How the calculation works

Pregnancy is conventionally dated from the first day of your last period, even though conception happens roughly two weeks later, which is why a full-term pregnancy is described as 40 weeks rather than 38. If you enter a conception date or IVF transfer date instead, the calculator works backwards to the equivalent last-period date, since this keeps the gestational week count consistent with how the NHS and maternity services describe pregnancy. Day 3 and day 5 (blastocyst) IVF transfers are handled slightly differently, since they represent different stages of embryo development at the point of transfer.

From that starting point, the calculator counts the number of days that have passed to reach today's date, converts that into whole weeks and remaining days, and checks which trimester and milestone band that falls into. Your due date itself is calculated as 280 days (40 weeks) from the same starting point, which is also how our pregnancy due date calculator works out your estimated delivery date.

Worked example

Say your last period started on 1 May 2026, with an average 28-day cycle, and today is 29 July 2026. That is 89 days since the first day of your last period, which works out to 12 weeks and 5 days pregnant, placing you right at the very end of the first trimester, with your dating scan likely either just completed or coming up very soon. Your due date would fall around 5 February 2027, roughly 191 days away.

Now try changing the calculation method to conception date in the calculator above, using a date around 15 May 2026 instead, roughly two weeks after the example last period date. You will see the calculator adjusts the equivalent starting point automatically, keeping your current week count and due date closely aligned with the last-period method, since both describe the same underlying pregnancy timeline from two different reference points.

Understanding your trimester and milestones

The first trimester runs from week 1 to the end of week 12, the second trimester from week 13 to the end of week 26, and the third trimester from week 27 until birth. Key NHS milestones along the way include your booking appointment, usually before 10 weeks, your dating scan around weeks 11 to 14, and your anomaly scan around week 20, which checks your baby's development in detail. Full term is reached at 37 weeks, meaning birth from this point onwards is considered a normal, full-term delivery rather than premature.

Common mistakes people make with pregnancy dating

A common mix-up is assuming pregnancy is dated from conception, leading to confusion about why a “40-week” pregnancy seems longer than the roughly 38 weeks a baby actually spends developing after conception. Another is relying on a single fixed 28-day cycle assumption when your own cycle is noticeably longer or shorter, which can shift your estimated dating by several days compared with what your scan later confirms.

It is also worth remembering that this kind of calculator gives an estimate based on typical timelines, not a medical diagnosis. If anything about your pregnancy feels uncertain, whether that is your dates, symptoms, or how you are feeling generally, your midwife or GP is always the right person to speak to, rather than relying on a calculator alone for reassurance.

Related calculators

If you are further back and want to estimate your due date from the very start, our pregnancy due date calculator covers that in full, including a complete list of NHS milestones. Our BMI calculator and TDEE calculator can help you understand general healthy weight and energy needs, though always follow your midwife's specific guidance on weight and nutrition during pregnancy rather than general calculator results.