When is that baby due?

When it comes to determining your due date, "things," as the Gilbert and Sullivan ditty goes, "are seldom what they seem." The methods of calculation are far from exact, common assumptions about the average length of pregnancy are wrong and calling it a "due date" is misleading. Understanding these uncertainties may help to curb your natural impatience to know exactly when labor will begin.

Many obstetricians want to induce labor when you exceed your due date by a set number of days, in the belief that prolonged pregnancy increases risk. As with dating the pregnancy, the evidence for inducing labor after a certain time past the due date isn't nearly as clear-cut as you might think, but that's another subject.) If induction were harmless, it wouldn't matter, but it's not. Among other adverse effects, inducing labor increases the odds of fetal distress during labor and cesarean section in first-time mothers, and mistiming the induction can result in a premature baby.

Page Two: Find out how long pregnancy really lasts

Page Three: Learn how accurate ultrasound is at setting due dates

Page Four: See what this means to you

Page Five: References

How long does pregnancy really last?

You might be surprised how the idea of a 40-week pregnancy came into being. In the early 1800s a German obstetrician simply declared that pregnancy lasts ten moon months counting from the start of the menstrual cycle prior to the pregnancy. (2) It took nearly 200 years for researchers to investigate whether this was, in fact, true. It turns out that it wasn't. When researchers in the late 1980s followed a group of healthy, white women with regular menstrual cycles, they discovered that pregnancy in first-time mothers averaged eight days longer than this, or forty-one weeks plus one day (2). The average was three days longer than forty weeks in women with prior births. The researchers also refer to other studies suggesting that other races may have average pregnancy lengths that are shorter than white women.

As you can see, the due date was only a probability that labor would begin sometime around that day. It was not a certainty, much less a deadline. Until recently, obstetric practitioners defined a full-term pregnancy as extending anywhere from 37 to 42 weeks. Today, many obstetricians call any pregnancy lasting to the beginning of week 41 "postterm." This, you will note, is one day less than the average length of pregnancy in first-time mothers.

Page Three: Learn how accurate ultrasound is at setting due dates

Page Four: See what this means to you

Page Five: References

How accurate is ultrasound at setting due dates?
A study has shown that an ultrasound isn't any more accurate than a reliable menstrual history combined with a pelvic exam by an experienced obstetrician. Researchers confirmed this by looking at pregnancies with known conception dates and comparing due dates arrived at by ultrasound measurements with dates arrived at by menstrual history and pelvic exam (4).

The fact that the old-fashioned method for dating a pregnancy does just as well as ultrasound is a vital point. While a sonogram may be useful in cases where there is uncertainty about when conception occurred, first-trimester sonograms are currently used as the ultimate standard. Your due date will often be changed if it differs from the one derived from the sonogram no matter how the date was previously determined or how sure you are of when you conceived.

Even first-trimester sonograms have a range of plus or minus five days, or a ten-day window, around the calculated date (3). The range increases to plus or minus eight days in the second trimester and plus or minus ten days for third-trimester scans. For this reason, experts say the due date should not be altered based on results from an early scan unless the calculated date differs by two weeks or more from the date determined by physical signs and symptoms and menstrual history (3).

Page Four: See what this means to you

Page Five: References

What does this mean to you?
The first lesson to be learned is have patience, unless there is a good reason not to wait for nature to take its course. When the fruit is ripe, it will fall from the tree. Inducing labor may be presented as a far more straightforward decision than it actually is. Think carefully about the risks and benefits of an induction recommended solely because you have not begun labor before some arbitrary cut-off date. Because it introduces risks, intervening in the natural process should only be undertaken to fix something that has gone awry.

Page Five: References

References