Pregnancy calculator
Gestational age is measured from the first day of the last period, roughly two weeks before conception actually happens. It survives because the period is a date most people can name and conception is not. It also means a pregnancy described as "four weeks" involves an embryo about two weeks old, which is why the dating scan, usually between 8 and 14 weeks, so often shifts the due date by a few days.
An estimate from a standard formula. Only a dating scan can date a pregnancy properly, and about one birth in twenty-five happens on the estimated date. The conception estimate carries a margin of several days, because ovulation timing varies and sperm survive up to five, so it must never be used to establish paternity or anything else that matters. Always follow the advice of your midwife or doctor.
A pregnancy due date is the first day of the last menstrual period plus 280 days. Gestational age is counted from that same date, about two weeks before conception — so a pregnancy at four weeks contains an embryo roughly two weeks old. Name the date you know and the panel returns the rest.
How to work out how far along you are
Naegele's rule has been in use since the early nineteenth century and is still the standard, though it carries two assumptions built into it. It assumes ovulation on day 14, which holds only for a 28-day cycle, and it assumes the cycle is regular. For a 35-day cycle ovulation falls nearer day 21 and the due date moves about a week later, which is the adjustment the cycle field makes. A first-trimester dating scan measures the embryo directly and beats any date arithmetic; where the two disagree by more than about five days, the scan is normally the one that stands.
Gestational age is the figure maternity care actually runs on, written as completed weeks plus days: 24w 3d is twenty-four full weeks and three days. Because it counts from the last period, it sits about two weeks ahead of the embryo's own age. Someone told she is six weeks pregnant is carrying an embryo of roughly four weeks, which causes real confusion against embryology diagrams, and it is the single most useful thing to understand before the first scan.
Conception is the least certain of the four figures. It is estimated at about 266 days before the due date, or roughly two weeks after the last period began, and the margin around it is several days wide: ovulation shifts between cycles, and sperm survive up to five days. So the date of intercourse and the date of conception are frequently different days, and a figure with that much slack in it proves nothing about either.
One label on the timeline repays reading carefully, because it is the one people act on. Until 2013 anything from 37 to 42 weeks was called term without further distinction, and a baby at 37 weeks was routinely described as full term. ACOG's Committee Opinion 579 split that band in four: 37 0/7 to 38 6/7 is early term, 39 0/7 to 40 6/7 is full term, 41 0/7 to 41 6/7 is late term, and 42 0/7 onwards is post term. The change was made because outcomes are not flat across those five weeks. Babies born at 37 and 38 weeks have more respiratory trouble, more feeding difficulty and more admissions to a neonatal unit than babies born at 39, and the old wording was being read as permission to deliver electively at 37. The due date view dates all four of those boundaries rather than one.
What people use it for
- Finding out how many weeks and days pregnant you are
- Working out a due date before a first appointment
- Estimating when conception happened from a due date
- Reading a maternity note written in weeks and days
- Checking a date given at a scan against your own dates
- Planning maternity leave around the full-term date
Questions
First day of the last period plus 280 days, or 40 weeks. A cycle longer or shorter than 28 days shifts it by the difference.
From the first day of the last menstrual period, in completed weeks plus days.
Twenty-four completed weeks and three days.
Because the last period is a date most people know and conception is not. Ovulation follows about two weeks later, so the count starts before the embryo exists.
Thirty-nine weeks. ACOG replaced the single 37-to-42-week band in 2013: 37 0/7 to 38 6/7 is early term, 39 0/7 to 40 6/7 is full term, 41 0/7 to 41 6/7 is late term, and 42 0/7 onwards is post term. All four are term rather than preterm; only the middle one is full.
No. It is the start of early term, and the point at which a birth stops being preterm. The distinction exists because babies born at 37 and 38 weeks have measurably more breathing problems, feeding difficulty and neonatal unit admissions than babies born at 39, which is why the older label was retired.
The first runs to the end of week 13, the second from 14 to 27, and the third from 28 until birth. The panel names it for you.
Around 4% of babies arrive on it. Most births fall within two weeks either side, which is why the whole 37-to-42-week span is treated as term rather than early or late.
Enter your usual length; a 35-day cycle moves the due date about a week later than the standard formula gives. If your cycles are genuinely irregular the estimate loses most of its value, and a dating scan becomes the better source.
No. A dating scan measures the embryo directly and is more reliable. This is for the weeks before that scan, and for curiosity after it.
Usually between 8 and 14 weeks. It is the point at which a date based on your own arithmetic is often adjusted by a few days.
About 266 days before the due date, or roughly two weeks after the last period started.
Several days wide at best. Ovulation timing shifts between cycles and sperm survive up to five days, so treat it as a week rather than a day.
Often not. Sperm can survive several days, so conception can happen well afterwards.
No, and it should never be used that way. The margin of error is far too wide.
Yes. Choose the due date mode, enter it, and the implied last period, conception date and current gestational age all follow from it.