Estimating your due date for Pregnancy

Pregnant? Estimate your Due Date

Your estimated Due Date is given to you by your Doctor. A normal full term pregnancy can last between thirty eight to forty two weeks, that is why it is only an estimate. Hence, only 4% of women give birth on their due date.

The duration of the pregnancy is 265 days on average from fertilisation to full term delivery. The difficulty is that the day of fertilisation is not always known. Often the first day of the last normal menstrual period is used because that day is more easily identified.

The Estimated Due Date is calculated by taking the first day of the last normal menstrual period, add seven, then count backwards three months.

As the pregnancy proceeds the Doctors are able to review their Estimated Due Date as their are additional milestones that they can look out for, such as:

  • The first fetal heartbeat can be heard using a special stethoscope at about 10 to 12 weeks. See our Pregnancy Calendar for more details on what happens in the pregnancy week by week.
  • The second indicator is based on using ultrasound. For example most women may have an ultrasound around the eleven to thirteen weeks, where they measure the height of the fundus, which is the top of the uterus, this is accurate to within about seven days.

If you have a regular menstrual cycle the Estimated Due Date and the date of delivery are more likely to be close. If you have an irregular menstrual cycle there is a greater chance that your delivery date will be later than your Estimated Due Date.

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