Every woman in the third trimester is counting down the days until baby is born.   Maybe the countdown begins out of excitement or maybe out of concern, but the waiting can be difficult.  Just how long should you be pregnant?

All of the pregnancy books I have read said that normal human gestation lasted 40 weeks but rarely is any child born on its due date.  My daughter was the exception 🙂  My son was five days late.

When planning a home birth, 36 weeks is the magic date for safe delivery depending on your midwife.  Many a mother, including myself, waits patiently for this day when experiencing signs of early labor.

The opposite holds true when you pass the magic 40 weeks mark.  At 41 weeks, doctors and midwives may start ordering tests and encouraging an induced labor.  Interventions are the norm.

Just what is normal gestation?   Research conducted in Great Britain suggests this five week variance is normal and not such a cause for concern.  The Independent reports:

Although it has always been known that human pregnancies usually last between 37 and 42 weeks, the variation has previously been attributed to imperfect methods of estimating due dates.

A woman’s due date is calculated as 280 days after the onset of her last menstruation and is only ever considered a best guess. Only four per cent of women deliver on their due date and only 70 per cent within 10 days of it.

However researchers from the National Institute of Environmental Health Sciences (NIEHS) were able to pinpoint the moment women became pregnant by taking daily urine samples – enabling them to isolate the role that natural variation plays in pregnancy length.

In a group of 125 women, they found the average time from ovulation to birth was 268 days – 38 weeks and two days – and that the length of pregnancies varied by as much as 37 days, according to the research, published today in the journal Human Reproduction.

This research should be used to ease the stress women feel about their due dates.  Five weeks is a long time for variation, and too often interventions are performed that were not necessary.   There are many examples of women who had an induced labor only to discover the baby and placenta showed no signs of being over term.

The Royal College of Midwives (RCM) said the research reaffirmed professionals’ experience “that every woman is different” and urged practitioners to be cautious intervening too early in pregnancy, but said that due dates were still useful.

Mervi Jokinen, practice and standards professional advisor at the RCM said: “I do not think that this signals the end of midwives and other health professionals giving a woman a single due date. It should be explained to the woman that the due date is always an estimate and as this research and our experience shows, this can vary widely.”