Estimated Date of Birth: How Overdue Are You?

You can expect to be asked “when are you due?” This line often becomes standardised greeting from family, friends or the chatty retail assistant. You may have already calculated your estimated date of birth using a phone app or online calculator and perhaps you’ve even had a scan revealing your ‘dates’.

The estimated due date of birth (EDB) forms a considerable role in the journey of a woman’s pregnancy. From a medical perceptive, it provides a gauge for growth and development and can aid toward a timeline of planning for relevant screening or diagnostic assessments. For women, it provides an anticipated time of baby’s arrival.

The EDB can determine the place of birth and management of labour, as well as expose the woman and her baby to an array of intervention. “Only four per cent of babies arrive on their ‘due’ date yet this date is perceived as a “critical piece of information” by midwives and doctors, and often by themselves” (Wickham, 2012, location 264).


I have expanded into detail below if you want to learn more about the in’s and out’s of estimated due dates however, if you’re short on time, I promise you this – when you go into spontaneous labour, you’re baby will arrive exactly when your baby arrives, is that not exciting?!

There is really no predicting this date. Many women get barraged by exuberant friends and family who call, text, knock on the door and Facebook stalk women who are expecting. Women find the constant checking from others exhausting. Can you imagine how much easier it would be to tell people you’re due about a fortnight after your due date? This will avoid the madness of responding to everyone who wants to know when and how far and if yet and blah blah blah arrrrggghhhh.

More importantly, unless you are having a planned operative birth, your baby is considered ‘full-term’, therefore likely to arrive between 37 and 42 weeks of pregnancy. A few babies arrive ‘pre-term’ (early or before their due) and some babies arrive ‘post-term’ (after 42 weeks of pregnancy). The message here is if you’d like to go completely nuts, focus on the due date, indeed count down in days and cross them off a calendar like a jailed inmate – don’t even think about this time being a window of expectation because that would be far more comfortable and who would want to be relaxed around the end of their pregnancy, wouldn’t you agree?

Continue reading if you’d like a more detailed explanation…


For eons of time the cycles of the moon and the menstrual cycles of a woman have been used to estimate the arrival of her newborn. During the 1800’s the boys got involved and similar to many medical methods, named after one of these blokes, we now have Naegele’s Rule. Variations of this curious little method have been used to this present day, even though the accuracy of which is critiqued.

For example, it has never been expressed whether we are supposed to consider the last menstrual period date (LMP) for the beginning or the end of the blood loss, although the first day of the LMP appears to be widely accepted. There are variations on the ‘rule’ from Naegele, who originally counted seven days from the LMP and added nine months to estimate the due date of birth. This is also written as adding seven days from the LMP and subtracting three months. At some point it was realised that this calculation was affected by the inclusion of the shortest month of February therefore 280 days from LMP is the calculation now utilised in pregnancy dating calendars.

More recent studies suggest that a woman having her first baby has a gestation (duration of pregnancy), of 288 days – whilst a woman who has already had a baby will have a gestation of 283 days.


You are a unique individual and this includes your menstrual cycle. Ovulation is where the egg comes down the fallopian tubes to be fertilised. This occurs 14 days plus or minus two days (12-16 days) prior to the first day of menstrual bleeding. A full length of cycle ranges from 15 – 45 days, averaging 28.1 days. Days of blood loss ranges from four to six with the most blood loss occurring on day two. When fertilisation occurs a woman can have an ‘implantation bleed’ (when the tiny fertilised egg is burying itself into the uterus, in order to get a solid grip). This can happen up to 12 days following sex. The woman experiences fresh red blood loss like a light period and although it doesn’t last long, is often mistaken as a ‘true’ period.

The absolute date of implantation is only ever certain for women who conceive with in vitro fertilisation (IVF) techniques, although the length of gestation following this has not been determined considering most women who conceive via IVF are offered induction of labour at, or prior to, forty weeks gestation.

It can be challenging to determine the first day of your last menstrual cycle based on memory unless you recorded it in a diary or on a calendar, along with previous menstruations. Most women, unless actively trying to conceive, don’t usually go to these lengths.

Let’s consider women who are breastfeeding and haven’t noticed any blood loss. With an approximate rate of 98 per cent for the first six months post partum, a woman who is exclusively breastfeeding day and night may experience breastfeeding amenorrhoea (no periods). Therefore it is possible she will ovulate before she experiences blood loss and not have an LMP to share.

Trying to figure out the estimated date of birth is one of the reasons that the current paradigm of modern obstetrics has justified the use of early ultrasound scanning. It is believed that women used to say they were having a ‘spring or summer’ baby. Fancy that?!


Scanning now forms an integral part of antenatal assessment and most women will experience one or more during pregnancy. Although I would never suggest it as necessary and many women choose not to scan their pregnancy, early USS is considered a helpful tool for many reasons including:

  • When women have no idea when their LMP was
  • To determine if there is more than one baby growing, although women will inevitably find out
  • If there has been a previous ectopic pregnancy (where the fertilised egg settles in a fallopian tube rather than the uterus) and would like to confirm the egg is now in the uterus
  • It forms part of early screening for differences (such as Down Syndrome)

Research shows us that routine ultrasound hasn’t not ‘improved’ the outcome for babies and there is ongoing debate about the safety of USS. Let’s consider the ethical aspect of ultrasound screening whereupon “women may feel compelled to participate in tests which are offered to them, believing that professionals know best” (Gail Thomas, 2004, p. 5, para. 2). Antenatal screening in pregnancy may be the first time women experience the concept of medical ‘risk’ and it’s important you are empowered to make your own decisions with regards to whether you choose to scan or not. This may indeed be the first time during pregnancy you are challenged or inspired to trust this natural process of your body; especially if you know your LMP, therefore estimated date of birth. You may be inclined to ask yourself what is the purpose of USS and will I learn anything more by having one?

So what’s your due date? When are you due? Or would you prefer to hear “what is your due window of time?” If that’s not fluffy enough for you, “are you having a Summer or Winter baby? A February or August baby?” Whew, (wipes beads of sweat from brow), that feels easier already does it not?

Gail Thomas, B. (2004). The disempowering concept of risk. In S. Wickham. Best Practice (pp. 3-5). Edinburgh, UK: Elsevier

Wickham. S. (2012). Induction: do I really need it? AIMS. [Kindle version]

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About KeiShana Coursey

My dance includes mother, lover, friend, yogini, midwife and HNLP practitioner. I shimmy through all those roles and depending on the beat, sometimes find myself face down on the floor with my buttocks exposed. (Usually at a time I’d like to be held up in the water - like ‘Baby’ in the movie Dirty Dancing). My life work is dedicated to an understanding of the mind as an incredible, sometimes slippery dance floor; how to continue to get up elegantly and dance with even more fervour. My theme songs are fuelled with passion for women's health, particularly the extraordinary steps that are pregnancy, labour, birth and parenting. These writings I offer you with open heart so you too may add to your own sweaty, muscle aching, arm swinging, feet stomping rhythm of life.