How do I calculate my estimated due date?

The short answer

For a first estimate, count 40 weeks, or 280 days, from the first day of your last menstrual period. The result is approximate. It gives you and your care team a reference point, but it cannot tell you exactly which day your baby will be born. This is the method described in Quebec’s Mieux vivre avec notre enfant pregnancy guide.

How do I calculate it?

Find the first day of your last menstrual period. Then add 40 weeks on a calendar, or count 280 days from that date. You can also enter the date into a due date calculator to get an estimate. Check that you have entered the first day of your period, not the last day.

The starting point is not the day you think you conceived. The exact time of fertilization is rarely known, so pregnancy is counted from the first day of the last menstrual period instead. As a result, the weeks of pregnancy include the first two weeks after your period, even though conception may not have happened yet during that time. This is a way of measuring pregnancy; it does not mean that fertilization took place on the first day of your period.

The common calculation assumes a regular 28-day cycle. If your cycles are longer, shorter or irregular, a calculator’s result may not fit your situation as well. Quebec’s Mieux vivre avec notre enfant guide, available as a PDF, explains this assumption. A calculator can still provide a starting point, but it should not be treated as a confirmed date for your pregnancy care.

An estimate, not an appointment in your calendar

Your estimated due date is a reference point, not a promise that labour will begin that day. Pregnancy length can vary, and this calculation cannot predict the exact day your baby will arrive. The date is mainly useful for placing your pregnancy in time and helping organize your care.

Keep in mind that the estimate depends on the information entered. If you do not remember when your last period began, are unsure of the date, or have irregular cycles, the calculation may be less useful. There is no need to compare your estimate with someone else’s. Each pregnancy has its own care and timeline.

At an appointment, your doctor or midwife may ask when your last period began and how long your cycles usually are. This information can help estimate how many weeks pregnant you are and your due date. If you are not certain, you can share the date you remember and explain that it may be approximate. Your health care provider can take that uncertainty into account. Quebec’s pregnancy follow-up guide describes the information that can be useful during prenatal care.

What is an ultrasound for?

An ultrasound can help estimate how far along your pregnancy is and your estimated due date. According to Mieux vivre avec notre enfant, an ultrasound done before 20 weeks can give a good idea of the date, with an accuracy of about 7 to 10 days. This is still an estimate; it does not guarantee the day your baby will be born.

The ultrasound estimate and the date calculated from your period may not be the same. If you are given a different date from the one you worked out, ask your doctor or midwife to explain which date will be used for your care. You do not have to decide between the dates on your own. Your care team can explain the reference point they are using and answer questions about it.

What should I note before my appointment?

To make the conversation easier, you can gather a few details before your appointment:

• The first day of your last period, if you know it.
• How long your cycles usually are, if they tend to be regular.
• The date of an ultrasound, if you have already had one.
• Any estimated dates from a calculator, especially if they differ from the date given at your appointment.
• Questions about how many weeks pregnant you are or which date will be used for your care.

If you do not know when your last period began, simply tell your health care provider. That information can be considered when estimating the date. You do not need to guess in order for your pregnancy care to begin. It can also help to mention whether your cycles are usually regular or whether you are unsure about the dates you remember.

When should I ask for help?

If you have a question about your pregnancy or something is worrying you, contact your doctor or midwife. In Quebec, you can also call Info-Santé at 811 for health advice. In an emergency, call 911. A calculator can offer a reference point, but it does not replace pregnancy care or advice from a health care professional.

The main takeaway: start with the first day of your last period and count 40 weeks to get an estimate. If your cycles are irregular, you do not know the date, or an ultrasound gives you a different reference point, discuss it with your care team.

Frequently asked questions

This article is for information; it does not replace advice from a health professional. In Québec, call Info-Santé 811 when in doubt (elsewhere in Canada, your provincial health line). In an emergency, call 911.

Sources

1. Tiny Tot to Toddler — complete guide (PDF), Institut national de santé publique du Québec (INSPQ), Mieux vivre