While Buckingham Palace never released an official due date, only stating that Prince William and Duchess Catherine's baby was due in "mid-July," the entire world has been discussing possible due dates, even betting on when the baby will arrive.
For this reason, taking a closer look at the obstetric concept of the due date and what it means is a good idea. Here's my explanation of the term:
Given the fact that only approximately 5 percent of babies arrive on their precise due date, it's no surprise when a baby is overdue. Full-term pregnancy refers to a five-week period of time between 37 weeks and 42 weeks, although babies born after 40 weeks (which represents the official due date) are considered post-term.
(Fun fact: in Obstetrics, the due date is technically referred to as the EDC, which stands for "estimated date of confinement." This harkens back to a time when pregnant women would need to remain house-bound to deliver their babies and then recover, thus they were "confined" to home!)
The important issue with projecting an estimated due date has to do with knowledge of dates roughly 10 months prior. This refers to the date of the last menstrual period, and length of a woman's menstrual cycle. For women who know the precise date of the period that occurred prior to conception, and who have a menstrual cycle that is 28 days long (many have cycles that are less or more than 28 days), and who know when conception occurred, it's significantly easier to accurately date the pregnancy and thus arrive at a valid due date. However, it's not only important for a woman to know these dates, but also crucial to have a sonogram in the first trimester of pregnancy, which measures the length of the fetus, the size of the gestational sac and can forecast the EDC.
A sonogram performed in the first trimester is said to be accurate to within a week, when done in the second trimester within two weeks, and within the third trimester within three weeks. Thus, the accuracy of an ultrasound to provide information on the size of the fetus that can project the due date is greatest within the first trimester.
So given the complex interaction between dates of a last menstrual period, measurement of a fetus that is just millimeters in size in the first trimester and the five-week range of time wherein a fetus can be born and be considered full-term, it's not surprising that many babies pass their projected due dates. When this happens, the fetus is usually monitored a little more closely with sonograms to check the amount of amniotic fluid surrounding the baby and other parameters. Called a Biophysical Profile, this non-invasive test checks the baby's heart rate in reaction to its body movement, it's muscular tone, its breathing movements (fetal lungs and chest expand and make breathing movements in utero) and its body movements.
The fetus can receive a maximum of two points for each of the five parameters, which help to test fetal well-being. A score between eight and 10 out of a maximum of 10 points is considered reassuring, although the Ob literature suggests that this test is really better at identifying a compromised fetus (that needs to be delivered) than it is at assessing a fetus that is "well." In other words, a low score may be more significant and predictive of an impaired fetus than a high score is of predicting a healthy fetus.
We also recommend the mom monitor fetal movement with "kick counts," which is a simple way of following how often the baby moves in utero. Though there is less room for a baby to kick at the end of pregnancy, babies in utero should still be moving at least 10 times in two hours. It's all about watching the clock at this point in pregnancy…watching and waiting.