Post term pregnancy and why a best before end date would be better!
I thought this would be a great subject for my first blog, primarily because it comes up all the time as a midwife, but also because my friend just went through this and despite daily contact and reassurance from me, I still found her wondering if she would ever go into labour once she hit 9 days late (she did, just a few hours later!)
“And just remember most first babies are late, a lot of them quite considerably late”. If I had £1 for every time I have said this to a pregnant woman I wouldn’t need to work! So, what is the deal with your ‘due date’ and why don’t babies come when they are supposed to?
First things first, we ought to consider the history. The idea that all gestations are the same length comes from a Dutch Professor way back in 1744 who looked at just 100 records of pregnant women and deduced that 9 months and 7 days should be added to their last menstrual period (LMP) date. What he didn’t declare is whether that is the first or last day of the period.
Later in 1812, a German Obstetrician Franz Karl Naegele repeated this statement, but still didn’t specify whether to use the first or the last day of the period! Finally, in 1872 another obstetrician named Gunning Bedford, this time in the US, also suggested using this approach and attributed it to Naegele, but this time clearly documenting it as from the ’termination of the last menstrual period’ i.e. the last day.
Despite this, textbooks in the US and UK from about the mid 19th century onwards began citing Naegele’s rule as from the first day of the last menstrual period. A lot of confusion! When you then take into consideration that all women vary in terms of how many days their period will last and that both menstrual cycles and calendar months vary in length, we find ourselves with a fairly inaccurate calculation of a due dates!
More recently, Naegele’s rule has been modified to 40 weeks or 280 days rather than 9 months and 7 days which addresses the problem of variation in month length. Furthermore, due date calculators such as the one on www.nhs.uk will allow you to adjust your cycle length thus solving the second problem. However, all this is still based on a pregnancy length suggested way back in 1744 from just 100 sets of notes!
With the introduction of ultrasound scanning to date pregnancies in the 1970’s, reliance on LMP dates has decreased and yet so often I hear of due dates being changed upon scan that make the implied date of conception ‘impossible’. The perinatal institute; a not for profit organisation that seeks to improve outcomes in UK Maternity care, take the dilemma of dating pregnancies very seriously and advocate a policy of scan dating particularly when it is within 7 days of the LMP date, as this is known to reduce the incidence of post term pregnancy.
However, having established how to reasonably accurately predict a due date we next need to consider what this actually means. In the UK we use the term EDD, which is short for Estimated Day of Delivery. Estimated. What this really means is ‘use of this date means we can accurately identify how many weeks and days you are’. This is really useful for screening in the first trimester as hormone levels change daily and measuring growth and plotting centiles in the second and third trimester and most importantly considering when an induction of labour might be considered. What it doesn’t mean is that there is any confidence that your baby will be born on that date. Humph. In fact, only 4% of babies are born on their due date! It is much more accurate to suggest that there is a 5 week window (37 – 42 weeks) during which your baby may be born that is known as “term”.
There is variation in length of pregnancy according to ethnic group, sex of baby, whether this is your first baby amongst other things and as I said right back at the beginning of the post - most babies are born late. In fact, a large study of over 30,000 pregnancies (Smith 2001*) showed 50% of first time mums were still pregnant 5 days past their due date, 25% at 9 days past their due date and 10% at 14 days past their due date. France have gone someway to addressing this by giving a ‘due date’ of 41 weeks rather than 40. The last time I checked (recently - I currently have a French lady) there are no physiological differences between French and British women, but what this means is that over 50% of mums in France will in fact have given birth ‘early’. Speaking from personal experience and on behalf of all of my women, I can confidently report that every day of being overdue is not only mildly disappointing, but also accompanied by the near constant requests of ‘have you had the baby yet’ from well meaning friends, family, neighbours etc. All this pressure and ‘expectation’ to be met can leave you wondering if you will ever go into labour naturally which is both stressful and undermines your faith in your body and yet your baby has no idea he/she was ‘supposed’ to be born already.
So, while in the mean time I urge my women to both expect to be very late and start lying to their friends and family (Midwife’s orders!) about when your due date is, wouldn’t it be better to just use a best before end date and allow most women to have their babies on time or early? Radical thoughts from the Positive Birth Midwife.
*Smith, G. C. (2001). Use of time to event analysis to estimate the normal duration of human pregnancy. Human Reproduction 16(7): 1497-1500.