Pregnancy timeline
Estimate a due date from your last menstrual period, a known conception date, or an IVF transfer, and see the gestational age today alongside the standard term window.
This is an educational estimate. Only your midwife, doctor or ultrasound report can confirm your dates.
Quick reference
Gestational age is counted from the first day of the last period, not from conception — which is why a pregnancy is considered two weeks along at fertilisation and why a normal pregnancy is described as about 40 weeks rather than 38.
| Stage | Weeks (from last period) | What it usually means |
|---|---|---|
| First trimester | 0 – 13 weeks 6 days | Organ formation; dating ultrasound is most accurate in this window |
| Second trimester | 14 – 27 weeks 6 days | Anatomy scan typically around 18–22 weeks |
| Third trimester | 28 weeks onward | Growth monitoring and birth planning |
| Early term | 37w 0d – 38w 6d | Near term; slightly higher risk than full term |
| Full term | 39w 0d – 40w 6d | The optimal window for most births |
| Late term | 41w 0d – 41w 6d | Increased monitoring is common |
| Post term | 42w 0d and beyond | Care teams usually discuss induction |
How dating works
The conventional estimate adds 280 days — 40 weeks — to the first day of the last menstrual period. This is Naegele's rule, and it survives because it needs nothing more than a date most people already track. The catch is baked into the arithmetic: the rule assumes a 28 day cycle with ovulation on day 14, so a cycle that runs long or short shifts the estimate. This calculator adjusts by the difference between your cycle length and 28, which is why entering an accurate cycle length matters.
If ovulation or conception is known — through ovulation tracking, insemination, or a single act of intercourse around ovulation — the estimate becomes more direct: 266 days from conception, because the 40 week total includes the roughly two weeks before conception occurs. Conception dating removes the cycle length guesswork, but only if the ovulation date is genuinely known rather than inferred.
In vitro fertilisation gives the most precise starting point available, because the age of the embryo at transfer is known exactly. A day 5 blastocyst is transferred already five days old, so the due date falls 261 days later; a day 3 cleavage stage embryo uses 263 days. Adding the embryo's age at transfer is what keeps IVF dating consistent with the 280 day convention.
An early ultrasound measures the embryo or fetus directly and, in the first trimester, estimates gestational age within roughly a week. Because that measurement reflects the pregnancy itself rather than a remembered date, obstetric guidance generally treats first trimester ultrasound dating as the most reliable basis, and many care teams will revise the calculated due date when the two disagree by more than a set threshold. A later scan is less useful for dating, because normal size variation between babies begins to swamp the signal.
Only around 4 percent of babies are born on their calculated due date. Roughly 80 percent arrive within the two weeks either side of it, and first pregnancies tend to run slightly longer than later ones. It is more useful to read the due date as the centre of a window than as an appointment: the term table above is the part worth planning around, and your care team's monitoring schedule is the part that turns it into decisions.
Gestational age is written as weeks plus days, so 24 weeks 3 days is not "24.3 weeks" — the trailing figure is days. Counting from the last period is the same convention your scan reports use, which is why this page reports both the due date and the age today in that format. The two must agree: if your scan says otherwise, the scan is the number your care team will work from.
Using the result
Most antenatal schedules are built backwards from the due date, and many workplaces ask for a date when arranging parental leave. Because the date is an estimate, it is sensible to plan around the 37 to 42 week window rather than a single day, and to keep documentation of the scan-derived date if your employer or insurer needs one.
It does not assess the health of a pregnancy, interpret symptoms, screen for risk, or recommend any clinical action. It performs calendar arithmetic on the dates you supply and reports the standard gestational timeline. Anything beyond that belongs with a clinician who can examine you and read your notes.
FAQ
It is a midpoint, not a prediction: about 4 percent of babies arrive on the exact date, and around 80 percent within two weeks either side. A first trimester scan generally gives a better estimate than the last period alone.
Naegele's rule assumes ovulation on day 14 of a 28 day cycle. If your cycle is 32 days, ovulation likely happens later, so the due date shifts about four days later — which is what the cycle adjustment handles.
No. It is counted from the first day of the last period, which is roughly two weeks before conception. That is why 40 weeks is the standard figure even though a pregnancy from conception is about 38 weeks.
Ultrasound dating from the first trimester is generally treated as more reliable, and care teams often revise the estimate to match it. Treat the scan as authoritative for appointments and planning.
It can estimate an ovulation or conception date by counting back about 266 days from the due date, but that is an inference from the same arithmetic, not a measurement — and it cannot identify a particular day of conception.
Yes. All calculation happens in your browser. Dates you enter are not transmitted to or stored on any server.
Important
This calculator provides an educational estimate of a due date based on the dates you enter. It is not a medical device, does not provide medical advice, and must not be used to make decisions about your pregnancy or your baby's health. Due dates are commonly revised after ultrasound. Always discuss your dates, symptoms and care plan with a qualified healthcare professional, and contact your maternity care provider promptly with any concern — do not wait for a calculated date.