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Anxious about when your baby is due?

As soon as you announce you’re pregnant, one of the first things your delighted audience usually demands to know is when you are due. The magic ‘due date’ is a big focus of pregnancy but, much to the frustration of many expectant parents, it’s an arbitrary one on which your baby is actually very unlikely to arrive. And the unpredictability of due dates can throw families into chaos, particularly if the rest of their world is tightly scheduled.
“My husband has to book leave six months in advance, so he took five weeks leave starting the week before Lily was supposed to be born,” recalls Rachel, who is married to a Department of Defence officer. “But Lily arrived two weeks late and we spent 10 days in hospital with a minor complication. He only had three days at home with us before he was posted overseas for four months.”
Welcome to the world of babies, where nothing is predictable, time is meaningless and there is no place for diaries or alarm clocks.
Calculating your ‘due date’ Sydney obstetrician Dr Gregory Davis admits the standard calculation of your ‘due date’ isn’t really very reliable and is subject to huge variability.
He says only 3 to 5 per cent of babies are actually born at 40 weeks, so it’s pretty uncommon to have your baby on the day you’re due.
“To calculate it add seven days to the first day of your last menstrual period, then nine months to the month,” Dr Davis explains. “So, if your last period started on the 7th of May, then your due date is going to be the 14th of February, which works out to be around 40 weeks.”
He says this method assumes the woman has a 28-day menstrual cycle, and ovulated and fell pregnant on day 14, so gestation is 38 weeks from the day of ovulation.

A more accurate estimate
Dr Davis admits the common ‘dates’ method of working out a due date is quite unscientific, and ultrasound can be more helpful, at the right time.
“An early ultrasound, between 8 and 12 weeks, will give quite an accurate estimate of plus or minus four or five days,” he says. “But later in pregnancy, ultrasound is not as accurate in assessing gestation because the baby’s growth is much more variable.”
Dr Davis says that while early scans are not routine, around 50 per cent of private obstetric patients in Sydney have an early scan to confirm dates. “A scan is far more accurate, particularly if you have an irregular cycle.”

Going early or going over
Dr Davis says women’s focus on the due date becomes a big concern nearing the end of the pregnancy. “Most women, by the time they get to the end of the pregnancy, become much more concerned about going overdue.” But, he points out, while the standard calculation of your ‘due date’ estimates gestation at 40 weeks, babies are most commonly born at 40 weeks and 3 days. “You’re statistically far more likely to go overdue than under.” He says it’s a common myth that women usually go later with their first baby, and he’s never seen reliable statistics to support that. “Usually, you do what you’ve done the first time – so if you’ve gone late the first time, you’ll go late the second time, and if you’ve gone early the first time, you’re more likely to go early the second time.”
Dr Davis says there’s no research showing that women follow their mother’s or sister’s pregnancies either. He says it’s good to remember that ‘term’ covers the period from 37 to 42 weeks, and unless there’s a problem, any baby born in this time is fine.

Time to induce
“The general recommendation is that if labour doesn’t happen spontaneously, we’ll do an induction between 41 and 42 weeks,” says Dr Davis. “Inducing before 41 weeks is more likely to end in a caesarean, because women fail to get into labour properly. After 42 weeks, the baby is more likely to go into distress because the placenta isn’t coping.”
He believes older women are just as able as younger women to have successful deliveries up to 42 weeks, although he says older first-time mums tend to be a little more anxious and there is more pressure for induction.
“There’s lots of criticism about obstetricians performing inductions, but unfortunately around 90 per cent of my overdue patients pressure me to induce them sooner. Most are desperately keen to see their baby.”
Despite this, Dr Davis says it’s far better for the mother and the baby if a woman goes into labour on her own.
“There’s no doubt that if you induce omeone, they’re more likely to have an epidural and then they’re more likely to need a vacuum or end up with a caesarean. It’s much better to be patient and let the baby come when it is ready.”

Letting nature take its course
Independent WA midwife Sam Mansfield says she tries to prepare her clients from the start for the idea that up to 42 weeks is still normal. “Induction definitely increases your chances of things going wrong, so it’s good if you don’t have to force your body into labour.”
She says another good strategy is to give family and friends a later date, so you are less likely to get endless phone calls asking if you have had the baby.
“If you tell your family and friends your due date is whatever 42 weeks would be, that holds the phone calls and text messages at bay a little bit longer.”
Babies have their own agendas, she adds. “People worry so much about going overdue, but most of it is in your head. If you don’t have a 28-day cycle and you know when you ovulated, you can trust your own estimates a bit more than the arbitrary calculation that is used at the moment.”

Number crunching

  • About 8 per cent of Australian babies are born before 37 weeks, but less than 0.5 per cent of all babies are born before 32 weeks.

  • Premature births in Australia increased by 20 per cent over the decade between 1995 and 2005, while births overall rose by only 5 per cent.

  • Only 3 to 5 per cent of babies are born on their ‘due date’.

  • In 2006, 91.5 per cent of babies born in Australia were born between 37 and 41 weeks, while 1.2 per cent were born after 42 weeks.

  • One study on labour found that 3.5 per cent of labours happened spontaneously at 37 weeks, 9.5 per cent at 38 weeks, and 18.5 per cent at 39 weeks.