Due Dates Explored, Birth in the ‘Safe Zone’

Estimated Due Dates (EDD’s) are calculated with the use of ‘Naegele’s Rule’ (devised by Hungarian obstetrician Franz Karl Naegele in 1830). The beautifully simple equation starts at the first day of a woman’s last menstrual period and adds 40 weeks. These days a slight adjustment may be made after gauging a more accurate starting point at the first scan but even so EDD’s are very rough estimates and only about 5% of babies arrive when they are due. The average arrival time for first time mothers is 41 weeks 1 day and for second time mothers 40 weeks 3 days. 90% babies arrive during the safe-zone of 37-42 weeks, with boys tending to gestate several days longer than girls (yes, we mature faster even in utero!).

My first baby arrived just outside of the safe zone at 36.6 weeks, luckily he was totally ready and we had a dream birth, it was lovely. When it came to my second pregnancy, based on the trend for pregnancy length to decrease with consecutive children, I worried the baby would come too early… I needn’t have, he came at 39.6 and, at nearly a pound heavier than my first, had serious difficulty getting out- in fact he only just made it. Having done my research on factors that can influence pregnancy length since then I believe I know what the difference was between my two pregnancies that led to my first being quite early and my second staying in to a point that he was too big for me- and I know what I’d be doing in an attempt to steer another pregnancy into my personal safe-zone if one came about….

Sex. Levels of certain prostaglandins are key to the onset of labour and one source that can boost the levels in the vagina to that associated with birth is found in semen (where its function is literally to aid muscualr contraction to help the sperm reach the egg). This is part of the reason some women with other risk factors are advised not to engage in sex during the third trimester. Of course, when baby is term or if like me experience has shown that I need baby to arrive a little earlier it can also be a really good idea to utilise this nudge (as much as possible!). Similarly, nipple stimulation encourages the release of oxytocin which is elevated during labour. It’s effect on speeding up delivery has seen nipple stimulation actually being increasingly seen in birth plans but studies suggest that an overdose of oxytocin can cause foetal distress so women are encouraged to only ever stimulate one at a time! Some prostaglandins that definitely do not encourage the onset of labour though are those whose release is triggered by eating of the infamous pineapple! The presence of the enzyme bromelain does aid the production of prostaglandins but not those that act on a woman’s reproductive system. More prostaglandins than can have an effect though (take note those guilty of harranguing the pregnant pineapple juice fan) are produced in response to stress. In addition, stress raises heart rate and blood pressure which diverts blood from the uterus which can encourage it to contract leading potentially to labour. Group B Streptococcus (GBS: a common infection all women should be tested for in early pregnancy) can also lead to shorter gestation periods due to a release of prostaglandins. Completing a course of antibiotics where it is present lengthens the likely gestation period but may not work for or appeal to all women. It may be particularly prudent to ascertain the policy of your hospital when considering how to manage infections as many insist on maternal intra-venous antibiotics where a mother has tested positive, certainly, for GBS at any point during pregnancy (regardless of her status since treatment or at the time of birth) which will enable baby to have a safe passage through the birth canal in most cases. Where the mother is not labouring long enough to receive the full dose baby may be required to receive their own intra-venous antibiotics over their first few days of life. For women who have experienced problems with infection historically self monitoring of vaginal PH and ‘yogurt treatment’ (that is literally finding a way to get natural yogurt in there, I find the kind of syringes they give with kids medicines now really useful for this!) has proven to be effective for many women.

An increasing issue for British women is stretching and weakening of the uterus which can lead to earlier births. This can be as a result of a natural malformations, fibroids, an excess of amniotic fluid, previous abortions or, crucially, multiple pregnancies (twins, etc.) which are increasing in incidence as a result of IVF (in-vitro fertilisation). The adoption of specific policies in fertility treatment such as limiting the number of embryos implanted during transfer has been shown to decarease the number of IVF babies born prematurely. Multiples (as well as the shape of a woman’s uterus in some cases) can also lead to poor circulation in the pelvis which again increases the likelihood of early babies. Where under 10% of singleton babies are born preterm over 50% of twins are. Women with a history of loss connected with natural malformations often find that Progesterone Therapy (which relaxes the muscles in the uterus, maintains cervical length and has anti-inflammatory properties) can maximise the length of pregnancy they can achieve. Another procedure that can be apt is Cervical Cerclage in which a suture is fitted around the cervix to hinder shortening. Other, more drastic, surgical procedures can also help before conception.

Many aspects of a mothers physiology can influence the length of her pregnancy, age features with both young mothers (defined as >16 at the time of conception) and women over 35 being more likely to give birth earlier. The size of a mothers natural build also impacts. This is because size affects metabolism, there being a limited number of calories any one person can burn in a day and during pregnancy women approach that ceiling, tending to give birth just on the cusp of reaching it. Smaller people have the potential to burn fewer calories so tend to birth younger (crucially smaller) babies. Heat raises metabolism so a heat wave can be particularly bad news for some and saunas should be avoided at all costs. The length and regularity of a woman’s menstrual cycle can effect the likely due date with women having longer or irregular cycles having longer pregnancies (that’s even when calculations are adjusted from LMP to measurements taken during scans). A 2003 study in London (which has since been backed up by several further studies) found that a mothers ethnicity was another indicating factor. It was found that Black and Asian mother’s birth dates averaged 39 weeks compared to White Europeans averaging 40 weeks. There were also higher rates of preterm delivery in the Black and Asian communities but, crucially, in these ‘preterm’ deliveries there was a higher rate of merconium (baby’s first poo) stained amniotic fluid indicating foetal maturity- so those babies were ready to enter the World. Speculative theories suggest that this is a result of Black and Asian women having smaller pelvis’ thus an imperative to get the baby out before it is too big to fit. This is this unproved however- and in societies such as Japan where high amounts of Omega 3’s (from fish) are consumed babies tend to gestate for longer than either of the groups mentioned in this study with no difficulty entering the World. Many maternal medical conditions such as Thyroid Disease, Diabetes, Asthma, Heart Disease and Pre-Eclampsia as well as being either underweight or overweight make it more likely that babies will be born early but it is unclear how much the figures are affected by safe, planned inductions. Certainly though, for women suffering with any medical condition it is wise to optimise their medical therapies prior to conception. Post conception frequent visits to medical professionals and as much education as possible lengthens the likely period of gestation across the board, for the most and least healthy of prospective mothers.

Lifestyle factors also have their effects: use of mouthwash lengthens (bizarrely), as does good massage by licensed therapists (poor massage often having the opposite effect) and good hydration (dehydration can actually cause contractions which can then be stopped with rehydration), rest- where appropriate- and, finally, more than one year (pre-conception) on folic acid supplements. Lifestyle factors that can shorten the length of pregnancy include: heavy/ physical work, long walks and standing for more than 6 hours per day (which all potentially stretch the uterus), air pollution, anxiety, depression and suffering abuse, carbon monoxide exposure (get your appliances checked and use alarms), night shift work, and working upwards of 42 hours per week.

The most beneficial action we can take for the safe delivery of our babies is educating ourselves as much as we possibly can. So read up on your risk factors, attend as many appointments as you can squeeze out of your healthcare team and don’t hesitate to question them, persist where brushed off (!) and flag up concerns.


4 thoughts on “Due Dates Explored, Birth in the ‘Safe Zone’

  1. Interesting. I’m an Black African woman of relatively small (slender) stature (132lbs/60kg at 5ft7/1.70m). I wonder now if that has any correlation with my baby being born at 38+6days weighing 5.9lbs/2.51kg. Tiny mommy, tiny baby?

  2. My two were exactly the same, takes so much pressure off when their gaining weight early. Well done with the breastfeeding too, I loved that- until my second was 16m and I was emaciated… It is such a huge commitment in terms of energy that you need to replace for your own health, I didn’t get that sorted second time round.

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