Your due date – what you need to know

Posted on August 21st, 2019

Midwife Pippa Hime explains how your estimated date of delivery (EDD) is calculated and factors that can affect it.

What you should know about your due date

As soon as you’ve found out whether you are pregnant or not, your next question is most likely to be, “When is the baby due?” It may come as a surprise to you, but your EDD is not an exact science. It is only an estimate as to when the baby will be born.

The most common, and easiest way, to work out your due date is to count 40 weeks or 280 days from the first day of your last period. This often confuses expectant parents as they are technically counting from before conception, but it provides a good estimate of when the baby is due. It is based on a normal, regular 28-day cycle, assuming that you have ovulated mid-cycle around day 14.

What if I don’t know my cycle?

Not all women are on a regular 28-day cycle. Some women have a longer cycle and may only ovulate on day 20, for example. Some women may not remember the date of their last menstrual period, especially if they were not planning to fall pregnant. If this is the case, the EDD comes from the first foetal ultrasound. Your obstetrician will measure the foetal sac and get an idea of how far along your pregnancy is from its size.

ALSO SEE: Calculate your due date with our easy calculator

Why would my EDD move?

Many factors can affect when a baby is ready to be born. However, on average, a pregnancy is anything from 38 to 42 weeks long. If your baby is big for its gestational age, your EDD may move forward. The number of babies you are carrying can also affect your estimated date of delivery. Multiples have an earlier due date than singletons, and triplets even earlier. The number of previous babies the mother has had can also play a role in determining when the current baby she’s carrying will be born. Most first-time pregnancies run past the due date.

ALSO SEE: What to do if you are overdue

When planning around your EDD, it is a good idea to rather look at it as a range of dates. Remember that only around 4% of babies arrive on their exact due date, so be prepared to be surprised!

Fun fact

A way to estimate how far along a woman is in her pregnancy, is to use a tape measure to measure the distance from the pubic bone over the top of the tummy to the very top of the uterus. That distance in centimetres is approximately equal to the weeks pregnant the woman is. This is known as MacDonald’s Rule.

Pippa Hime

About Pippa Hime

Pippa is a Registered Professional Nurse and trained as a Registered Midwife at Chris Hani Baragwanth Hospital. She has extensive experience in all things baby related with a special interest in preparing couples for the exciting journey of parenthood as well as supporting them in the weeks that follow the birth. She and her husband Richard are the proud parents of Becca age 6 and Tom age 4. Pippa has a comprehensive private clinic service that includes Childbirth Education classes, a Well Baby Clinic including Immunization as well as Post Natal and Lactation support. With over 5 years of running a private clinic facility and raising 2 children Pippa comes with a wealth of knowledge and first-hand experience of parenthood. Learn more about Pippa Hime