There are a few different ways to calculate a due date. Most often, your due date is calculated by counting 40 weeks from the first day of your last menstrual period (LMP). This is pretty accurate for determining when a baby is 40 weeks old, which is not the same as when you will go into labor!! The window for term babies is anywhere from 37 to 42 weeks. Your due date is just the 40 week benchmark. Nobody can accurately predict the day when you will have your baby, and only 5% of babies are actually born on their due date.
USING LMP TO CALCULATE DUE DATE:
Calculating a due date is done by counting 40 weeks from the first day of your last menstrual period (LMP.) Using the LMP is accurate only if you ovulated and conceived around day 14 of your cycle. Most women with a 28 day cycle do ovulate around day 14. However, if your cycle is shorter, then you probably ovulate and can conceive earlier. Likewise, if you have a long cycle (for example, every 35 days), then you will probably ovulate and conceive around day 21. If you have irregular cycles, then when you ovulate and conceived is even less predictable. When cycles are irregular, or long or short by more than a few days, an early ultrasound can help to narrow it down and get a more accurate due date. Click for more on “How pregnant am I?” Early ultrasounds (around 8 weeks) can establish the gestational age and a due date with an accuracy of plus or minus 4 or 5 days. REMEMBER, just because the ultrasound gives you a due date, that does not mean you will give birth that day! It just establishes when you will reach the 40 week mark.
ULTRASOUNDS AND DUE DATES:
An ultrasound in the first trimester can accurately establish the age of the pregnancy and the due date to within a few days by measuring the embryo from head to rump. After the first trimester, ultrasounds become less accurate in dating the pregnancy. Ultrasound doesn’t tell you how old the baby is- rather it uses the SIZE of the baby to estimate the age. As the weeks go by, using size becomes a much less precise method since the normal variability in size from one baby to another begins to widen. Like children and people, in utero some babies are larger or smaller than their peers of the same exact age. For example, one baby can weigh 9 lb. on its due date, while another only weighs 7 lbs, despite being conceived on the same day. The average size at term is about 7.5 lbs. However, a larger than average baby is going to measure ‘older’, even though it’s not. Thus, the later the ultrasound, the more likely the baby will be expressing its inherited tendency to be petite or large, and the less useful it is in dating the pregnancy. So, don’t get excited that your due date ‘moves up’ if your 32 week ultrasound shows an earlier due date! Big babies don’t necessarily come earlier, nor are there lungs ready sooner. On the other hand, a smaller baby will look like its due date is pushed farther back. However, small babies shouldn’t stay ‘in’ longer— in fact, if growth is poor, that might mean the baby needs to be born early. Due dates established early in the pregnancy do not change, despite what your later ultrasounds show.
Click for more on later ultrasounds and growth.
WHY ESTABLISH A DUE DATE?
Hopefully we have made the point that due dates do not predict when labor and birth will happen. So why bother making a ‘due date’? Knowing the age of the baby at any given week and the 40 week mark is very important because the accuracy of many of the screening tests that you will have depends on them being performed at specific weeks in the pregnancy. Additionally, it helps your doctor or midwife make decisions about how to treat you should you go into preterm labor, or if you go too far beyond your due date. Going more than 1 to 2 weeks beyond your due date significantly increases the risks, making it prudent to do more careful monitoring such as non-stress tests and biophysical profiles, and eventually inducing labor.