Women's Health
Pregnancy Calculator
Find out exactly how far along you are, which trimester that puts you in, and how much time is left.
Pregnancy Calculator
These dates are estimates from an average cycle. Real cycles vary month to month, so treat every result as approximate and confirm anything you are relying on with a midwife or doctor.
About the Pregnancy Calculator
Enter either the first day of your last period or your due date, and this calculator tells you exactly how far along you are — in weeks and days, which is how maternity care counts.
It shows which trimester that puts you in, how many days remain until your due date, and a rough size comparison for the current week. It also converts weeks into months, which sounds simple and genuinely is not.
The counting convention surprises many people: pregnancy is dated from the first day of your last period, roughly two weeks before conception. So when you are "6 weeks pregnant", the embryo has existed for about four weeks.
How to use the Pregnancy Calculator
- Choose whether you know your last period date or your due date.
- Enter that date.
- If using your last period, enter your average cycle length so the estimate can be adjusted.
- Select How far along am I?
- The bar shows how far through the 40 weeks you are.
How gestational age is counted
Gestational age = days since the first day of your last period
Weeks and days = total days ÷ 7, with the remainder as days
Written as "12w3d" — twelve completed weeks and three days.
Worked example
Last period began 3 September; today is 20 November.
Days elapsed: 78.
78 ÷ 7 = 11 weeks with 1 day remaining.
11 weeks and 1 day — first trimester, due around 10 June
Why pregnancy is counted from your last period
Because it is the one date that can be known reliably. Conception happens internally and usually without any noticeable sign, whereas the first day of a period is unmistakable and easy to remember.
The consequence is that gestational age runs about two weeks ahead of the embryo's actual age. At the moment of conception you are already counted as two weeks pregnant, and a positive test typically arrives at around four weeks — when the embryo is roughly two weeks old.
Clinicians call the embryo's true age fetal age or conceptional age, but almost every date you are given in maternity care — scans, tests, appointments — uses gestational age. When comparing figures, check which one is meant.
The three trimesters (trimester calculator)
| Trimester | Weeks | Length |
|---|---|---|
| First | 0w0d – 13w6d | About 14 weeks |
| Second | 14w0d – 27w6d | 14 weeks |
| Third | 28w0d until birth | About 12 weeks |
Trimester boundaries vary slightly between sources — some place the second trimester start at 13 weeks, others at 14. This calculator uses the 14-week convention, which is the most widely used. A difference of a few days at a boundary carries no clinical meaning.
Weeks to months, and why it does not divide neatly
Pregnancy is 40 weeks, which is about 9.2 calendar months — not 10 months, and not 9 tidy ones. Calendar months average 4.35 weeks, not 4, so dividing weeks by four overstates the month.
| Weeks | Month | Trimester |
|---|---|---|
| 1 – 4 | 1 | First |
| 5 – 8 | 2 | First |
| 9 – 13 | 3 | First |
| 14 – 17 | 4 | Second |
| 18 – 22 | 5 | Second |
| 23 – 27 | 6 | Second |
| 28 – 31 | 7 | Third |
| 32 – 35 | 8 | Third |
| 36 – 40 | 9 | Third |
So 30 weeks is month seven, not "seven and a half". This is why maternity care works in weeks throughout and largely ignores months — weeks are unambiguous, months are not.
What term means
- Preterm — before 37 weeks.
- Early term — 37w0d to 38w6d.
- Full term — 39w0d to 40w6d.
- Late term — 41w0d to 41w6d.
- Post-term — 42 weeks and beyond.
These categories were formalised because outcomes differ measurably across them, which is why "term" is no longer treated as a single block from 37 weeks onward.
If your due date changes after a scan
This is common and usually means nothing is wrong. If a first-trimester scan disagrees with your period-based date by more than about five to seven days, the scan date is adopted, because early ultrasound dating is more reliable than a remembered period date.
Once revised, the new date is used for everything that follows — screening windows, growth expectations and any discussion about induction. The due date calculator covers all four dating methods, and the conception calculator works backwards to a likely conception date.
This is an estimate, not medical advice. It works from average cycle figures, and real bodies vary a great deal month to month. Nothing here replaces a consultation. For anything you are relying on — a pregnancy, fertility treatment, a health concern, or a decision about contraception — speak to a midwife, doctor or sexual health service who can assess your own situation.
Pregnancy wheel calculator online
Doctors and midwives have long used a cardboard pregnancy wheel. You line up the first day of the last period, and the wheel shows the due date and how many weeks along you are. This calculator does the same job, without the small print or rounding errors. It also accounts for your cycle length, which a basic wheel ignores.
If I'm X weeks pregnant, when am I due?
A pregnancy lasts about 40 weeks. Subtract how far along you are from 40, then count that many weeks forward from today.
Due date ≈ today + (40 − weeks pregnant) × 7 days
| Weeks pregnant today | Weeks to go | Due in about |
|---|---|---|
| 5 weeks | 35 | 8 months |
| 7 weeks | 33 | 7.5 months |
| 8 weeks | 32 | 7.4 months |
| 12 weeks | 28 | 6.4 months |
| 20 weeks | 20 | 4.6 months |
| 29 weeks | 11 | 2.5 months |
For a due date from a scan or IVF transfer, use the pregnancy due date calculator.
Corrected gestational age for premature babies
If a baby is born early, doctors track growth using corrected age: the baby’s age since birth minus the weeks they arrived early. A baby born at 32 weeks (8 weeks early) who is now 5 months old has a corrected age of about 3 months. Doctors usually use corrected age until about age 2.
Other ways to use this calculator
Need an IVF due date calculator? Use the pregnancy due date calculator, which has a dedicated IVF and frozen embryo transfer option. For a gestational age calculator from an ultrasound, the same page lets you enter the scan date and the age the scan gave.
Pregnancy weeks and dates from any starting point.
- Pregnancy week calculator — enter your last period or due date to see weeks, days and trimester.
- IVF and FET pregnancy calculator — see IVF and FET due dates.
- Pregnancy weeks calculator — how many weeks pregnant am I? Enter your last period date.
- Weeks pregnant due date — the result shows both your current week and your due date.
Frequently asked questions
Because the first day of a period is a date you can actually know, while conception happens internally and usually unnoticed. The consequence is that gestational age runs about two weeks ahead of the embryo's real age — at conception you are already counted as two weeks pregnant, and a positive test usually appears around four weeks, when the embryo is roughly two weeks old.
The first trimester runs from 0w0d to 13w6d, the second from 14w0d to 27w6d, and the third from 28w0d until birth. Sources differ slightly — some start the second trimester at 13 weeks rather than 14 — and this calculator uses the 14-week convention. A few days either way at a boundary has no clinical significance.
Month seven. Calendar months average about 4.35 weeks rather than 4, so dividing weeks by four overstates it — 30 weeks is not seven and a half months. Weeks 28 to 31 make up month seven. This mismatch is exactly why maternity care counts in weeks throughout and largely avoids months altogether.
That is common and normally means nothing is wrong. If a first-trimester scan differs from your period-based date by more than about five to seven days, the scan date is adopted, because early ultrasound dating is more accurate than a remembered period date. The revised date is then used for screening windows, growth expectations and any decisions near term.
About 33 weeks from today, since a pregnancy lasts around 40 weeks. That is roughly seven and a half months away. Your first scan may move the date by a few days.
About 6.5 to 7 months. You are in the third trimester and in the seventh month of pregnancy. Dividing weeks by 4.35, the average number of weeks in a month, gives the closest answer.
Sources and further reading
- ACOG — Methods for estimating the due date (Committee Opinion 700) — gestational age conventions and redating
- ACOG — Definition of term pregnancy — the early/full/late/post-term categories
- NHS — You and your baby, week by week — what happens at each stage of pregnancy
- NICE — Antenatal care guideline (NG201) — UK antenatal schedule and dating practice