Glengarry Private Hospital
Part of Ramsay Health Care

How to Work Out Your Due Date

Newly pregnant and wondering when your baby will arrive?

Working out when your baby will come into the world is probably one of the first things you will do as an expectant parent. Even if you’re not currently pregnant, due date calculators are also very useful to help you plan for any future arrivals.

A nine-month pregnancy, so it’s often called, seems easy enough to work out. Start with the first day of your last period and then count 40 weeks (or 280 days) into the future. Or, you can work out your due date with this calculator.

There are some things to note about this, though. Ovulation and conception are assumed to have occurred around day 14 of your cycle, but we include those two weeks between your period and egg fertilisation. This means the baby is in utero for 38 weeks total, not 40. You also may have noticed I said ‘nine-month pregnancy’ but a lot of women are surprised to find out this is nine full months.

Medical professionals refer to it as Estimated Date of Confinement (EDC) or Estimated Date of Delivery (EDD). Pregnancy naturally varies by five weeks, so a full-term pregnancy is between 37 and 42 weeks. To break it down a little more, this is between 259 days and 294 days – a 35-day variance. This no doubt frustrates heavily pregnant women patiently counting down to their EDD.

How accurate is a due date?

As you can see, with a five-week range for a full-term pregnancy it can be difficult to predict the exact date of birth. How often do babies actually arrive on time? Not very often. It is estimated that only five per cent of babies are born right on the EDD. In Australia, 91 per cent of babies are born between 37 and 41 weeks, 8.6 per cent are pre-term, and less than one per cent are post-term – more than 42 weeks.

While it may seem strange that we still use a dating system that is such a rough estimate, it’s because it is the easiest and most accurate method in the beginning of a pregnancy. It’s worth noting though that not all countries use this system, which is based on a principle called Naegele’s Rule. Some countries, including France, count 41 weeks as a typical full-term pregnancy, not 40.

Factors that interfere with a due date

Your cycle isn’t the ‘standard’ 28 days. Naegele’s Rule works on the principle that a menstrual cycle is 28 days long, with ovulation occurring around day 14. As many women know, this is certainly not the case for everyone, as cycles can range between 20 and 40 days. If you have a short cycle you may deliver earlier, and if you have a long cycle you may deliver later. Or, you may have irregular cycles which makes timing even more difficult. As an added layer of confusion, your cycle can fluctuate with stress, weight changes and excessive physical activity.

You don’t ovulate on day 14. Because of the variations in cycle length, you could have ovulated as early as day eight or as late as day 20. And ovulation can vary, even if you have a ‘normal’ 28-day cycle. If ovulation and fertilisation occurs on a different day, then your EDD may change too. Talk to your obstetrician if you’re unsure.

You recently stopped your contraceptive. When you decide to finish up contraceptives, be aware that your fertility (ovulation) may sometimes take time to return. This is very individual. It also differs depending on what method you were using. For example, those on ‘the Pill’ should find their fertility returns relatively quickly or even straight away, whereas those on the hormone injection Depo Provera could be waiting six to 12 months for their periods to return.

Why do babies arrive early?

Babies do make their appearance earlier in certain circumstances. Pre-term births — those before 37 weeks — are common among mothers who are teenage, are more than 40 years old, smoke during pregnancy, live in remote areas, are pregnant with multiple babies, or are Indigenous. It can also be due to problems with the placenta, pre-eclampsia or growth restriction of the unborn baby. There is evidence that an unusual shaped uterus and infections are also factors.

Why do babies arrive late?

If you’ve had a prior pregnancy that was post-term, you may be more likely to have another. We aren’t sure why babies are overdue but it may happen if it’s your first baby, if the baby is a boy or if you’re obese. Of course, it may also be that the due date wasn’t calculated correctly.

The more accurate method

You can have an ultrasound scan between 10 and 14 weeks, although it may be offered as early as six weeks if you’ve experienced bleeding or previous miscarriages. This scan can confirm how far the pregnancy has progressed and confirm whether your initial due date is approximately correct. It will also confirm whether you are pregnant with one baby or more and will check the baby’s development and movement. This is also the time when screening can be done for Down’s syndrome and spina bifida.
As frustrating as it can be when you’re patiently waiting for your baby’s arrival, remember that the specific day is not what is important. An estimated due date is not only an end date; it is also a helpful way to keep track of mother and baby’s progress and health.

About The Author

Dr Stas Vashevnik is a private gynaecologist and obstetrician at Glengarry Private Hospital. He has a keen interest in complex deliveries and in discovering the underlying cause of recurrent miscarriage. To learn more about Dr Vashevnik, visit his website