Learn how your due date is calculated from your last period, why early ultrasound can change it, and what term, trimesters and 37 to 42 weeks really mean.
Finding out you are pregnant brings one question almost immediately: when is the baby coming? Your estimated due date helps you and your care provider plan prenatal visits, screening tests and the timing of your care. But it is an estimate, not a deadline. Here is how it is worked out, and why it can shift.
You can get a quick estimate with our free due date calculator, then read on to understand what the number means.
The starting point: your last menstrual period
Most due dates begin with the first day of your last menstrual period (LMP). The classic method, known as Naegele's rule, adds 280 days (40 weeks) to that date. A shortcut version: take the first day of your last period, add one year, subtract three months and add seven days.
This means pregnancy is counted from before conception actually happens. In a textbook 28-day cycle, ovulation happens around day 14, so at "two weeks pregnant" you have only just conceived. This can feel confusing, but it gives everyone a shared, consistent way of counting.
Adjusting for your cycle length
Naegele's rule assumes a 28-day cycle with ovulation around day 14. Many people have longer or shorter cycles. Because the phase after ovulation tends to be fairly steady at around two weeks, a longer cycle usually means later ovulation, and a shorter cycle means earlier ovulation.
A simple adjustment is to add the number of days your cycle is longer than 28, or subtract the days it is shorter. For example, with a regular 32-day cycle, you might add about four days to the standard estimate. Our calculator lets you enter your usual cycle length for this reason.
LMP dating is less reliable if:
- your cycles are irregular or you are unsure of the date
- you conceived soon after stopping hormonal birth control, or while breastfeeding
- you had bleeding in early pregnancy that may have been mistaken for a period
If you conceived through IVF, your clinic will date the pregnancy from the embryo transfer date instead, which is very precise.
Why a dating ultrasound may change your date
An ultrasound in the first trimester is the most accurate way to date a pregnancy. Early on, babies grow at a very similar rate, so measuring the length from crown to rump gives a reliable estimate of gestational age.
If the ultrasound estimate differs from your LMP date by more than a set amount, your care provider will usually change your due date to match the ultrasound. Once a due date is set from a reliable early scan, it generally should not be changed by later ultrasounds, because babies vary more in size as pregnancy goes on.
Don't worry if your date moves after your first scan. It simply means your care team now has better information.
The short version
Very few babies arrive on their due date
Only a small share of babies, often estimated at around 4 to 5 percent, are born on their exact due date. Most arrive in the weeks around it. It can help to think of a "due month" or due window rather than a single day, especially when making plans for leave, family visits or travel.
What "term" means: 37 to 42 weeks
Pregnancy is considered term between 37 and 42 weeks. Within that window, care providers use more specific labels:
- Early term: 37 weeks 0 days to 38 weeks 6 days
- Full term: 39 weeks 0 days to 40 weeks 6 days
- Late term: 41 weeks 0 days to 41 weeks 6 days
- Post-term: 42 weeks 0 days and beyond
Babies born before 37 weeks are considered preterm. These categories matter because outcomes differ slightly across them, and your provider may recommend extra monitoring or discuss induction as you move past your due date.
The three trimesters
Pregnancy is commonly divided into three trimesters, each about three months long:
- First trimester (up to about 13 weeks): organs begin forming. Nausea, tiredness and tender breasts are common. This is when a dating ultrasound is usually offered.
- Second trimester (about 14 to 27 weeks): many people feel more energetic. A detailed anatomy ultrasound is often done around 18 to 22 weeks, and you may start feeling movement.
- Third trimester (about 28 weeks to birth): the baby gains most of its weight. Visits become more frequent as you prepare for birth.
When to get medical advice
A due date calculator is a planning tool, not a replacement for prenatal care. Book a visit with your family doctor, midwife or obstetrician as soon as you think you are pregnant. Seek care urgently, or go to your nearest emergency department, if you have:
- heavy vaginal bleeding, or bleeding with cramping or pain
- severe pain in your abdomen or shoulder, especially on one side, in early pregnancy
- fluid leaking from the vagina before 37 weeks
- regular contractions or pelvic pressure before 37 weeks
- a noticeable decrease in your baby's movements later in pregnancy
- a severe headache, vision changes or sudden swelling of your face or hands
In British Columbia you can also call HealthLink BC at 8-1-1 any time for advice from a nurse. If you think it is an emergency, call 9-1-1.
Try the free due date & pregnancy week tool
Your estimated due date, how far along you are and key dates ahead. It takes about a minute and your answers stay on your device.
