Even obstetricians become concerned, despite evidence that human gestation may be longer than 40 weeks and varies widely due to race, the length of a woman's menstrual cycle and other factors.
Of course, most women are sick of being pregnant after nine months. They are heavy and exhausted long before their due dates, and going overdue becomes almost unbearable. It can produce overwhelming feelings of conflict, where yearning to have the pregnancy over battles with concern for the baby's safety.
I recently gave birth to my second child, who arrived 10 days after his Estimated Date of Delivery (EDD). By the time labour started spontaneously, I was so desperate that if somebody had suggested dancing naked in Trafalgar Square I'd have done it without hesitation.
My first child had been born four days after her EDD and, because babies are supposed to arrive earlier and weigh more each time, I was convinced I would deliver a huge early baby the second time. My overdue time was especially difficult because I experienced what one midwife described as 'false labour'. Most days I had contractions every 10 to 15 minutes, which would speed up to around five minutes for hours at a time - and then fade away.
Nicky Wesson, author of Home Birth, suggests several natural methods of starting labour. I tried most of them. They include:
Frequent sex: the prostaglandin hormones in semen help to soften the cervix and induce labour. Midwives often advise overdue women to try the 'hot curry, hot bath, hot sex' method of natural induction.
Nipple stimulation: twiddling your nipples (or getting someone to do it for you) stimulates the production of oxytocin, the hormone that promotes contraction.
Taking the herb golden seal in capsules or tincture; taking castor oil or homoeopathic remedies such as caulophyllum.
Cervical sweeps carried out by midwives, where the cervix is wiped during an internal examination.
However, Nicky Wesson stresses that these remedies will only kick-start labour if the woman is just about ready. And they will not work for everybody. My husband had high hopes for the acupuncture. It might have worked with more than one session. However, it just left me rather drunkenly relaxed and, inexplicably, making chicken noises in the car on the way home.
By the eighth overdue day I was seriously considering hospital induction. But I kept changing my mind, even minute by minute. Most days I would end up weeping with exhaustion and indecision. If it had not been for the reassurance of the community midwives that the baby just was not ready to be born, I would not have been able to wait for spontaneous labour.
In the end, after all the false starts and stops, I went into labour on Easter Monday at 8.10am precisely. I was sitting in the bath trying to decide whether we should go to Chessington Zoo when I started having strong contractions three minutes apart and had to rush to phone the midwife, start filling the birthing pool in the dining room and distract our toddler with a chocolate elephant. Daniel was born just under three hours later, with no wrinkles. I had no stitches and no drugs.
Women might accept fewer inductions and attach less importance to delivery dates if they knew the basis of the method used to calculate the date. Naegele's Rule, as the method is called, is being questioned, and research suggests it may always have allowed too short a period.
Naegele's Rule holds that a normal human pregnancy is nine months and seven days from the first day of the woman's last menstrual period; 10 menstrual cycles in length. This algorithm was first proposed by Hermann Hoehaave, professor of medicine and botany at the University of Leyden, and adopted by Franz Carl Naegele (1778-1851). WE Montgomery, the English obstetrician, writing in 1837, says women at that time calculated their due date based on a 42-week gestation.
Nigel Saunders, consultant obstetrician and gynaecologist at the Princess Anne Hospital, Southampton, says there has been 'no critical analysis of whether the Rule was any good'. Dr Saunders says that since it is an average, around half can be expected to deliver in the two weeks prior to it and half in the two weeks after. Only 5 per cent of women actually deliver as calculated by Naegele's Rule. He says that in one study carried out by Dr Linda Cardozo of King's College Hospital, London, in which women were allowed to wait for as long as it took for spontaneous labour to occur safely, one pregnancy lasted 45 weeks. He adds that he always stresses to pregnant women that the EDD is a 'rough guide'. Recent research tends to support the idea that pregnancy should be thought of as longer than 40 weeks. One American study in 1990 of 114 white women with 28-day menstrual cycles found that first babies were delivered, on average, at 41 weeks and one day and subsequent babies at 40 weeks and three days. The researchers wrote in a letter to the Lancet that Naegele's Rule could not be applied universally.
Other clinical research found that Japanese pregnancies were, on average, 264 days (37 weeks and five days) and black mothers had pregnancies 8.5 days shorter than white mothers of similar socio-economic backgrounds. In 1991, a study of Swedish births between 1976 and 1980 found that 10 per cent took place after 42 weeks.
Doctors also accept that the longer a woman's menstrual cycle, the longer the pregnancies are likely to be. Surprisingly few women are aware of this.
John Friend, spokesman for the Royal College of Obstetricians and Gynaecologists, did not think that pregnant women or their families would be happy with yet more uncertainty. 'Humans celebrate and fix on dates; for example, birthdays. I don't think many mothers would accept not being given a date,' he says. He also points out that as maternity pay and maternity leave entitlements were calculated based on the EDD, a range of dates could cause problems.
Melanie Every, of the Royal College of Midwives, says: 'A lot of doctors give advice based on previous experience. If they had a bad experience (with a post-term pregnancy) they will want to induce earlier rather than later.'
Dr Friend said that as the placenta is believed to become less efficient at providing the baby with oxygen after 40 weeks, there is a slight risk to the baby in longer pregnancies.
The Royal College of Midwives is working with the Royal College of Obstetricians and Gynaecologists to produce information leaflets for pregnant women, which are expected to be published later this year. The leaflet on induction will advise it after 42 weeks.
I am sure that the ease and speed of Daniel's arrival proved he was not ready to be born any sooner. I also believe that had I known more about post-term pregnancy, both I and my family would have coped much better.
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