Finding out you are pregnant brings a lot of questions, and one of the first is: when is my baby due? Doctors and midwives use a standard method to estimate the date. This guide explains how due dates are calculated, what “weeks pregnant” really means and why the date is an estimate rather than a promise.
This article is general information and not medical advice. Your doctor or midwife will confirm your due date and care plan.
Why Pregnancy Is Counted From Your Last Period
It may seem odd, but pregnancy is counted from the first day of your last menstrual period (LMP), not from the day of conception. Conception usually happens about two weeks later, but the exact day is hard to know. The first day of your last period is a date most people can remember, so it became the standard starting point. This is why you are already counted as “two weeks pregnant” around the time of conception.
Method 1: Last Menstrual Period (Naegele’s Rule)
Add 280 days (40 weeks) to the first day of your last period. A common shortcut is to add one year, subtract three months and add seven days.
Example: If your last period began on 1 January 2027, your estimated due date is 8 October 2027.
This method assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, you can adjust the date by the difference. For example, with a 32-day cycle, add four days.
Method 2: Conception Date
If you know when you conceived, for example after fertility treatment, add 266 days (38 weeks) to that date. This is the same as counting from a last period date that is 14 days before conception.
Method 3: Ultrasound
An early ultrasound measures the baby and estimates how far along the pregnancy is. Doctors often use it to confirm or adjust the due date, especially when periods are irregular or the date of the last period is not certain. Early scans, in the first trimester, are generally the most reliable for dating.
Which Method Is Best?
- Regular cycles and a known LMP: the LMP date is usually a good estimate.
- Irregular cycles or uncertain dates: an early ultrasound is usually more reliable.
- Assisted conception: the transfer or conception date gives a precise estimate.
Try our free pregnancy due date calculator, which supports all three methods.
Understanding “Weeks and Days”
Doctors write gestational age as weeks and days. For example, “12+3” means 12 weeks and 3 days. Pregnancy is divided into three trimesters:
- First trimester: weeks 1 to 12
- Second trimester: weeks 13 to 26
- Third trimester: week 27 until birth
What Counts as Full Term?
Many health organisations use these terms:
| Term | Weeks |
|---|---|
| Early term | 37 weeks to 38 weeks 6 days |
| Full term | 39 weeks to 40 weeks 6 days |
| Late term | 41 weeks to 41 weeks 6 days |
| Post term | 42 weeks and beyond |
Why Babies Rarely Arrive on the Due Date
A due date is an estimate at the middle of a range, not a deadline. Pregnancies naturally vary in length, and about 1 in 20 babies is born exactly on the estimated date. Many are born a week or two earlier or later, and both can be normal. Your doctor will monitor you if the pregnancy goes beyond the estimated date.
Can the Due Date Change?
Yes. Your doctor may adjust the date after an ultrasound if the baby’s size does not match the date from your last period. A change does not mean anything is wrong. It simply gives a more accurate estimate.
What to Do After You Know Your Due Date
- Book your first prenatal appointment with a doctor or midwife.
- Ask which tests and check-ups you will need and when.
- Follow your healthcare professional’s advice on nutrition, supplements and medicines.
- Plan ahead financially. Our monthly budget guide and sinking funds guide can help you prepare for the costs of a new baby.
When to Contact a Doctor
Contact your doctor or go to an emergency service if you have heavy bleeding, severe pain, reduced baby movements, fluid leaking, or any symptom that worries you. Do not wait for your next appointment.
Frequently Asked Questions
How accurate is a due date?
It is a good estimate, but the baby can arrive any time within a few weeks of it. Treat it as a guide.
What if I do not remember my last period?
Your doctor can use an early ultrasound to estimate the due date.
Is a due date from conception different from LMP?
The two methods use different starting points (266 days from conception and 280 days from the last period), but for a typical 28-day cycle they give the same date.
This article is for educational purposes only and does not replace advice from a qualified healthcare professional.