Working out your baby’s due date is simple – and complex!
Doing the maths
Due dates are calculated as 40 weeks from the first day of the last menstrual period (LMP) – or 38 weeks (266 days) from conception. Length of pregnancy is counted from the LMP, so four weeks after your LMP you would be counted as being four weeks pregnant even though you probably only conceived two weeks ago. If you know when your LMP was, the easiest way to work out your due date is to go through the calendar counting forward in weeks till you get to forty weeks. So if your LMP was on a Tuesday, then your due date will be on a Tuesday. Counting 38 weeks from conception is more accurate, if you know when it was, especially for women who do not have regular 28 day cycles. (Remember, conception, when the sperm combines with the egg, can occur several days after sex.)
Health care professionals do not have time to count through the calendar so they use one of two ways to quickly calculate your due date. The first is a wheel which has dates of the year on the outer wheel and the weeks of pregnancy on the inner wheel. By matching 0 weeks on the inner wheel to the LMP date on the outer wheel, the corresponding due date at 40 weeks can be read off. These wheels are also used to work out how many weeks pregnant you are at check ups. These wheels are not very accurate and can be out by a day or two.
The other way is to take the LMP and add on nine months and seven days; for example, LMP 7 Jan gives a due date of 14 October. Again this is an approximate measure and can be out by a day or two: in 2016 7th Jan is a Thursday and 14th Oct is a Friday.
The first scan routinely offered in the UK is a dating scan, at about 12 weeks. This is used to confirm a due date from LMP or to work one out if LMP is not known (as well as some checks for abnormalities). It does this by measuring the length of the femur, I believe. At this point in the pregnancy there is a good correlation between this measurement and predicted due date. As the pregnancy progresses it is harder to assess gestation in this way as babies growth varies. Later scan are less reliable in predicting size of baby too. Research has shown that due dates from dating scans are more accurate than from LMP. If, from this first scan, the due date calculated is more than a week different from that from LMP it is likely that the health professionals will change the EDD. This can be confusing – for one of my friends it meant that, by the scan dates, she would have conceived when her and her partner were in different countries!
How can there be such discrepancies?
Of course she could not conceive without a partner and scans are only approximate. It cannot be an exact science. Women have varying lengths of menstrual cycles and do not all ovulate on day fourteen of their cycle. Many women have unpredictable cycles. Sometimes events in a person’s life can cause ovulation to happen earlier or later than usual. You can decline the scan. If you are sure of your dates then you do not need it to confirm this. If you do not know your LMP and would like a scan to give you a due date then you can ask them not to check for abnomalities if you don't want to know (at this scan the nuchal fold is measured which is a marker for Down's Syndrome).
When will my baby arrive?
However a due date is calculated – by calendar, by wheel, by rough approximation, or by scan – a due date is only an approximate date for baby’s arrival; in fact it is often written as EDD (estimated due date or estimated date of delivery). Only 4% of babies arrive on their due date and ‘term’ is defined as any time between 37-42 weeks, so the baby could come any time 3 weeks before that date or up to 2 weeks after and still be considered normal – and some babies come earlier than 37 weeks and some later than 42! The average is actually 40 weeks and 3 days - and for first timers the average is 2 days more.
So does it matter?
At the beginning of the pregnancy it is hard to imagine that one or two days difference in a due date matters. We all like to have a date to consider. We also have a tendency to wish the pregnancy by and want the baby to come as early as possible. However, it is worth going with the latest due date from one of the methods – if the date given by midwife/gp/scan is later than from counting through the calendar, then keep quiet; but if it is earlier then insist on using your date. By having a later due date you give yourself a bit more time and at 41 weeks every day will count. You will feel pressure from yourself and others for the baby to come. (Some women choose to tell family and friends that baby is due ‘sometime in June’ rather than give a specific day.) Also in this country it is routine to offer induction to women to start labour off at 10 or 12 days after the due date. Induction can result in a more painful labour and is associated with a higher use of epidural and assisted birth (forceps/ventouse). If induction methods fail to get labour going or progressing then women will be given a caesarean. So the later your due date, the less pressure and worry on you – for a day or two – which could be all that baby needs to come on its own. (Of course you can decline induction – you can wait and be monitored.)
And then you wait ...
Of course all of this has no bearing on when your baby will actually be born. If anyone invented a way of giving women due dates that were accurate even to a day or two then they would be rich; instead we have to put up with not knowing and a 5 week window, which even then isn’t definitive. Baby chooses the day – and from my experience babies want you to know from the start who’s in charge – them!