Pregnancy Calculator
Find out how many weeks pregnant you are today, from whichever date you know.
Pregnancy Calculator — your inputs
- What do you know?
- First day of my last period
- Average cycle length
- 28 days
- Unit system
- Metric
Key takeaways
- Gestational age is counted from the first day of your last period, not from conception — which is why you are counted as about two weeks pregnant on the day you conceive.
- '8w 4d' means eight completed weeks and four days. You are in week nine. This one-off difference is behind most apparent disagreements between an app and a midwife.
- A first-trimester ultrasound is the most accurate dating method available, and it is what your maternity team will use if it differs from your LMP date by more than a few days.1
- Natural pregnancy length varies by around five weeks across individuals even when dating is precise — the due date is the middle of a distribution, not a deadline.2
- 'Term' is not a single day: 37–38w 6d is early term, 39–40w 6d full term, 41 weeks late term.4
How to use this calculator
Pick whichever date you actually know — last period, conception, an ultrasound result, an IVF transfer, or a due date you have already been given.
Enter that date. For the LMP method you can also adjust your cycle length, which shifts the estimate for longer or shorter cycles.
Read your gestational age in weeks and days, then open the week-by-week table to see every remaining week dated.
Understanding your result
Gestational age is counted from the first day of your last period, not from conception — which is why you are counted as roughly two weeks pregnant on the day you conceive.
'8w 4d' means eight completed weeks and four days. You are in week nine, which is why an app and a midwife can appear to disagree by one while saying the same thing.
Whichever date you start from, the calculator converts it to the same day zero and counts forward, so every method produces a comparable answer.
Why you were 'two weeks pregnant' before you conceived
This is the first thing that confuses almost everyone, and it is a bookkeeping convention rather than a claim about biology. Gestational age is counted from the first day of your last menstrual period — a date you can actually report — rather than from conception, which almost nobody can pin down. Since ovulation typically happens about two weeks after the period starts, the clock is already running for a fortnight before there is anything to count.
Everyone uses the same convention: your midwife, your scan report, your hospital notes and every pregnancy book on the shelf. That consistency is the whole point. The alternative — dating from conception, sometimes called fetal or embryonic age — runs exactly two weeks behind and appears mostly in embryology texts.
| Way of counting | At conception | At birth |
|---|---|---|
| Gestational age (what everyone uses) | 2 weeks | ~40 weeks |
| Fetal / embryonic age | 0 weeks | ~38 weeks |
| Months, roughly | Half a month | 9 months |
'8w 4d' versus 'week 9' — both are right
Gestational age is written as completed weeks plus days. At 8w 4d you have completed eight full weeks and are four days into the ninth. So you are in week 9 — in exactly the way you are in your thirty-first year when you are thirty years old.
This calculator gives you both, because both get used. Scan reports and clinical notes almost always use the completed form (8w 4d); apps, books and conversation almost always use the week you are in ("I'm nine weeks"). When a number seems off by one, this is usually why — not an error in either source.
Trimesters follow the week number
Weeks 1–13 are the first trimester, 14–27 the second, and 28 onwards the third. The boundaries are conventional rather than biological — nothing changes overnight on the first day of week 14.
Which dating method to trust
This calculator accepts five starting points, and they are genuinely not equal. Ranked by how much they can be relied on:
| Method | Typical accuracy | Why |
|---|---|---|
| First-trimester ultrasound (success) | ±5 to 7 days | Measures the pregnancy directly, when size varies least between individuals |
| IVF transfer date (success) | ±1 day | The date of fertilisation is known exactly, which is rare |
| Known conception date (info) | ±3 to 5 days | Removes the ovulation guess, but conception may not be same-day as intercourse |
| Second-trimester ultrasound (warning) | ±10 to 14 days | Individual growth differences have started to spread the measurements out |
| LMP with a regular cycle (warning) | ±1 to 2 weeks | Assumes you ovulated on schedule — an assumption the data does not support well5 |
LMP dating sits low on that list for a reason worth spelling out. It assumes ovulation on day 14 of a 28-day cycle, and the large-scale cycle data shows that neither half of that assumption holds for most people.5 The calculator adjusts for your cycle length, which helps — but it still cannot know whether you ovulated on schedule this cycle.
Your maternity team's date wins
Once your care team sets an official dating, use theirs. It is what your screening windows, growth scans and induction discussions are all built around, and having two dates in play is how appointments get missed.3
What the due date does and does not promise
Only about one birth in twenty happens on the estimated due date. That is not a failure of the estimate — it is what a single-day prediction of a naturally variable event looks like.
Jukic and colleagues measured this carefully, using daily hormone sampling to identify ovulation precisely and so remove dating error from the picture. Even with the conception date effectively known, the length of pregnancy still varied by around 37 days across individuals.2 The spread is real biology, not measurement noise.
- Preterm (34–37)
- Early term (37–39)
- Full term (39–41)
- Late term (41–42)
- Post term (42–43)
| Category | Range |
|---|---|
| Preterm | 34 to 37 weeks |
| Early term | 37 to 39 weeks |
| Full term | 39 to 41 weeks |
| Late term | 41 to 42 weeks |
| Post term | 42 to 43 weeks |
What this calculator cannot tell you
- Whether the pregnancy is progressing normally. It converts dates. It has no information about the pregnancy itself.
- When you will actually give birth. The due date is the midpoint of a five-week distribution.
- Anything about a multiple pregnancy. Twins and higher multiples are dated and managed differently, and are usually born earlier.
- How your baby compares to the week-by-week sizes. Those are population averages, and individual variation at every single week is wide. A difference from the table is not a finding.
Contact your maternity team, not a calculator
Bleeding, severe or one-sided abdominal pain, a sudden change in your baby's movements after you have started feeling them, severe headache with visual changes, or persistent vomiting all warrant contacting your maternity team the same day. None of these are things dates can assess.
Terms used on this page
- Gestational age
- Time elapsed since the first day of the last menstrual period, written as completed weeks and days (e.g. 8w 4d). The standard way pregnancy is dated everywhere.
- Day zero
- The LMP-equivalent start of the 280-day clock. Every dating method this calculator accepts is converted to a day zero first, so the answers stay comparable.
- Viability
- The gestational age from which survival outside the womb becomes possible with intensive care, conventionally marked around 24 weeks. Outcomes improve steadily from there rather than switching on.
- Full term
- 39 weeks 0 days to 40 weeks 6 days — the window associated with the best outcomes. Distinct from 'early term' (37–38w 6d), which is still term.
The formula
- LMP = 1 March 2026, 28-day cycle, so day zero = 1 March
- Today = 8 August 2026, which is 160 days later
- 160 ÷ 7 = 22 weeks and 6 days
Source: Gestational age from LMP-equivalent day zero. Published clinical/scientific formula.
Limitations
- LMP dating assumes you ovulated on schedule. If your cycles are irregular or you conceived soon after stopping hormonal contraception, the estimate can be off by more than a week.
- A first-trimester ultrasound is the most accurate dating method available, and once your maternity team sets an official date, that is the one your care is scheduled around.
- The week-by-week sizes are population averages. Individual babies vary widely at every single week, and a difference from the table is not in itself a finding.
References
- 1.Committee Opinion No. 700: Methods for Estimating the Due Date. American College of Obstetricians and Gynecologists, 2017 (reaffirmed 2021).
- 2.Length of human pregnancy and contributors to its natural variation. Jukic et al., Human Reproduction 28(10):2848–2855, 2013.
- 3.Antenatal care: NICE guideline NG201. National Institute for Health and Care Excellence, United Kingdom, 2021.
- 4.Committee Opinion No. 611: Method for Estimating Due Date (definition of term pregnancy). American College of Obstetricians and Gynecologists, 2013 (reaffirmed 2021).
- 5.Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. Bull et al., npj Digital Medicine 2:83, 2019.
Frequently asked questions
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Open calculatorRelated articles
How Far Along Am I? Reading Pregnancy Dates Without the Confusion
Why you were two weeks pregnant before you conceived, what '8w 4d' means, which dating method actually wins, and why your app and your midwife disagree by one week.
Read articlehCG Levels in Early Pregnancy: What the Numbers Do and Don't Mean
Why 'doubles every 48 hours' is only half true, why the 66% rule is out of date, and why a single hCG value tells you almost nothing.
Read articleNot medical advice. This calculator is for general informational purposes only and is not a substitute for professional medical guidance. Always consult a qualified healthcare provider before making decisions about your health. Read the full medical disclaimer.
Printed from mystatura.com — for general information only, not medical advice.