One of the most common misconceptions about pregnancy is that it lasts nine months when, in reality, the average pregnancy is 266 days or 38 weeks from conception. Medical professionals typically date a pregnancy in weeks to gain a more accurate and specific age of the pregnancy. They predict a due date by counting back three months from the first day of the last menstrual period (LMP), then adding one year and seven days. Pregnancies are broken up into trimesters: 0-12 weeks make up the first trimester, 13-24 weeks are the second trimester and 25-40 weeks constitute the third trimester.
It was only during the 20th century that predicting a due date became more of a scientific endeavor. While there were probably many “scientific” ways of estimating it in the past, there was no shortage of old wives' methods and incorrect medical theories. A 19th century obstetrician in Germany put forth the idea that a typical pregnancy could be measured by starting with the LMP and counting forward ten moon cycles. This was proven wrong as more modern studies were conducted.
Recent studies have shown that mothers carrying their first child have pregnancies averaging 41 weeks and one day. For subsequent pregnancies, the average duration is 40 weeks and three days. In general, healthcare professionals agree that all pregnancies, when using the LMP dating technique, average approximately 280 days or 40 weeks, based on a 28 day menstrual cycle. When predicting when the baby is due by using the LMP date, two weeks are added to the fetus’ age, because conception usually occurs two weeks after the start of the LMP. The reason the date is calculated from the LMP is because it is more difficult to pinpoint ovulation and conception than to determine the last menstrual period.
If a woman has an irregular menstrual cycle or can’t remember when the first day of her LMP was, an ultrasound is performed to help predict the due date. In this procedure, an ultrasound technician measures the fetus to help determine its age. Although an ultrasound performed in the first trimester is considered best for predicting the most accurate date, this method has an error rate of five days on either side. An ultrasound conducted in the second trimester has a plus or minus error rate of eight days, and a third trimester ultrasound has a plus or minus ten day error rate.
Even a healthcare professional’s predicted due date is nothing more than a prediction. Only 10% of women deliver on the estimated day, but half of women hit it within one week, and 90% deliver within two weeks of the predicted date. This is why some obstetricians have started predicting due “weeks” instead of days.
