How the Pregnancy Calculator works
The Pregnancy Calculator turns one date you know into a complete picture of your pregnancy. Enter the first day of your last menstrual period (LMP), your conception date, or your IVF embryo transfer date, and it works out your estimated due date, how many weeks and days pregnant you are today, which month and trimester you are in, how many days have passed and how many remain, and how far through a 40-week pregnancy you have come. It also builds a personal timeline so you can see the previous, current and next week at a glance.
Everything happens instantly in your browser. The dates you type are never stored on our servers, and there is nothing to sign up for. You can share or print your result, and come back to recalculate at any time — the numbers always update to today’s date.
What the Pregnancy Calculator tells you
| Result | What it means |
|---|---|
| Estimated due date | The day you reach 40 weeks of pregnancy — the middle of the usual window for birth, not a fixed appointment. |
| Pregnancy week and day | How far along you are, in completed weeks and days (for example 20 weeks 3 days). |
| Pregnancy day | Which day of pregnancy today is, out of roughly 280. |
| Pregnancy month | The approximate month (1 to 9), using the common grouping of weeks into months. |
| Trimester | First (weeks 1–13), second (weeks 14–27) or third (week 28 until birth). |
| Estimated conception date | When conception most likely happened — about two weeks after the start of the last period in a 28-day cycle. |
| Days pregnant and days remaining | Completed days of pregnancy, and the approximate days and weeks left until the due date. |
| Pregnancy progress | The percentage of a 40-week pregnancy already completed, shown on a trimester bar. |
All of these are estimates. They follow the same conventions used in prenatal care, but every pregnancy has its own timing, and your healthcare professional may adjust your dates — usually after an early ultrasound.
How pregnancy weeks are calculated
Pregnancy is not counted from conception. By convention it is counted from the first day of the last menstrual period, because most people know that date while the exact day of conception is rarely certain. In a typical 28-day cycle, ovulation and conception happen about 14 days after the period starts, so by the time conception happens you are already considered “two weeks pregnant”. This measure is called gestational age, and it is what doctors, midwives, ultrasound reports and this calculator all use.
The calculation itself is simple: today’s date minus the first day of your last period gives the number of days pregnant. Dividing by seven gives completed weeks, and the remainder is the extra days. If your last period started 143 days ago, you are 20 weeks 3 days pregnant (143 = 20 × 7 + 3) and today is day 144 of your pregnancy. The baby’s own age — sometimes called fetal or embryonic age — is about two weeks less.
Read more in our guide to how pregnancy weeks are calculated, or jump straight to the Pregnancy Week Calculator if all you need is how far along you are.
How the due date is estimated
The standard rule — known as Naegele’s rule — adds 280 days (40 weeks) to the first day of the last period. A quick calendar shortcut gives almost the same answer: add one year, subtract three months and add seven days. Because months have different lengths, the shortcut can be a day or two off, so the calculator always counts exact days, including leap years.
Estimated due date = first day of last period + 280 days
Conception date + 266 days · IVF day-5 transfer + 261 days · IVF day-3 transfer + 263 days
Worked example: if your last period started on March 2, 2026, adding 280 days gives an estimated due date of December 7, 2026. The estimated conception date is March 16, 2026, and the second trimester begins on June 8, 2026.
Cycle length matters. Naegele’s rule assumes ovulation on day 14 of a 28-day cycle. If your cycles are regularly longer, ovulation tends to happen later, and the due date moves later by the same number of days; shorter cycles move it earlier. Open “Cycle length” in the calculator to adjust it — a 32-day cycle adds four days, a 25-day cycle subtracts three.
From conception or IVF. If you know your conception date, the calculator adds 266 days (38 weeks). For IVF, the embryo’s age at transfer is known exactly, so it adds 266 days minus that age. For a full breakdown of every method — including dating from an ultrasound — use the Due Date Calculator, our second main tool.
Understanding the pregnancy timeline
A full-term pregnancy is divided into three trimesters. The calculator highlights where you are on this timeline and shows the previous, current and next week, each linking to a week-by-week page.
| Trimester | Weeks | Months (approx.) | Typical focus |
|---|---|---|---|
| First | Week 1 to 13 + 6 days | Months 1–3 | Confirming the pregnancy, dating, early screening |
| Second | Week 14 to 27 + 6 days | Months 4–6 | Anatomy scan, first movements, glucose screening |
| Third | Week 28 to birth (40 weeks ≈ due date) | Months 7–9 | Growth, position, birth preparation |
Within the third trimester there are further milestones: a pregnancy is early term from 37 weeks, full term from 39 weeks, late term from 41 weeks and post-term from 42 weeks. The estimated due date sits at exactly 40 weeks. For a complete week-by-week calendar with your own dates, open the Pregnancy Timeline.
Pregnancy week vs pregnancy month
People often ask “how many months pregnant am I?”, but pregnancy does not divide neatly into calendar months. Forty weeks is about nine calendar months and one week, and each month is roughly 4.3 weeks, so different books and apps group weeks into months slightly differently. This site uses the common nine-month grouping below. Healthcare professionals almost always use weeks and days, because they are more precise — a week makes a real difference to which tests are offered and how growth is assessed.
| Pregnancy month | Weeks pregnant (completed) | Trimester |
|---|---|---|
| Month 1 | Weeks 1–4 | First |
| Month 2 | 5 to 8 weeks | First |
| Month 3 | 9 to 13 weeks | First |
| Month 4 | 14 to 17 weeks | Second |
| Month 5 | 18 to 22 weeks | Second |
| Month 6 | 23 to 27 weeks | Second |
| Month 7 | 28 to 31 weeks | Third |
| Month 8 | 32 to 35 weeks | Third |
| Month 9 | 36 to 40 weeks | Third |
The Pregnancy Month Calculator shows your month alongside your exact week, and each month has its own guide, from month 1 to month 9.
How conception dates are estimated
The calculator places conception about 266 days (38 weeks) before the estimated due date — about two weeks after the first day of the last period for a 28-day cycle. In reality, conception can happen within a window of several days: sperm can survive for up to about five days, and the egg for around a day after ovulation. That is why the conception date is always an estimate, even when you remember the date of intercourse. The Conception Date Calculator shows the likely fertile window as well.
How accurate is a pregnancy calculator?
A pregnancy calculator is as accurate as the date you give it and the assumptions behind it. When cycles are regular and the LMP date is certain, an LMP-based due date is a good estimate, and IVF dates are the most precise of all. Even so, only a small minority of babies — often quoted at around 4 to 5 percent — are born on the due date itself. Most arrive within about two weeks either side, and birth anywhere between 37 and 42 weeks is considered within the usual range.
An early ultrasound, ideally in the first trimester, measures the embryo and is generally the most reliable way to confirm gestational age. If the scan and your LMP disagree by more than a certain margin, your provider may change your due date to match the scan. Our guide How Accurate Is a Due Date? explains when and why that happens.
What can affect an estimated due date
- Irregular or long cycles. Ovulation may not happen on day 14, so an LMP-based date can be early or late.
- An uncertain period date. Spotting mistaken for a period, or a period that is hard to remember, shifts every result.
- Recent hormonal contraception or breastfeeding, which can make the timing of ovulation less predictable.
- Early ultrasound measurements, which may lead your provider to revise the date.
- Individual variation — first pregnancies, previous pregnancy lengths and many other factors influence when labour begins.
Tips for getting the most useful result
- Enter the first day of real bleeding, not the last day of your period and not spotting.
- If your cycles are consistently longer or shorter than 28 days, set your cycle length.
- If you already have a due date from a scan, use the Pregnancy Week Calculator in “Due date” mode so every result matches your provider’s dating.
- Use Share to save a link to your result, or Print to keep a copy with your pregnancy notes.
Whichever result you see, it is general information. Your midwife, doctor or other healthcare professional can confirm your dates and give guidance that fits your own pregnancy.