The Low Down on Due Dates

So, what is your due date anyway? It’s an estimate of when your baby will be born based on the first day of your last menstrual cycle calculated by adding 280 days to that date. This number would bring you approximately to 40 weeks of gestation. It assumes a 28 day menstrual cycle and a normal time of ovulation.

Some providers might calculate a more accurate date based on other details that you provide, particularly if you have an irregular cycle or know the day that you most likely conceived. Providers may further adjust the estimated due date with information gathered from early ultrasounds. A word of caution to parents about this: early ultrasound measurements (in the first trimester) are the most accurate for this. The margin of error for these measurements is often cited as approximately 2 weeks, however as gestation progresses the margin of error increases. One study states that for measurements taken from ultrasounds at 17-36 weeks, errors in estimation of gestational age could be as much as ten weeks. To put that in perspective, a trimester of pregnancy is between 12 and 14 weeks.

The 40 week estimate of birth falls roughly halfway within a range of normal. Babies are considered “term” at 37 weeks of gestation, however they are not considered “post dates” aka ‘overdue’ until 42 weeks gestation. Yup, you read that right. You aren’t out of the range of normal gestation until after 42 weeks. Even then, we must allow for the fact that some healthy individuals may still fall outside this range and have a normal birth and a well baby. This considered, some pregnancies carried past 42 weeks are not healthy, and most providers using evidence based practices will increase surveillance around 41 or 42 weeks depending on other factors in the pregnancy.

So, when will you give birth? There isn’t any way to know when your body will naturally go into the process of labor. Some recent study suggests that there are more factors than healthcare professionals first assumed that contribute to when labor begins. These factors include (but are not limited to) variable time of implantation, early hormone levels, age, weight, genetic factors, and events in the first two weeks of pregnancy. It has also been stated that factors traditionally thought to influence length of gestation such as BMI, alcohol intake, parity (how many prior pregnancies), and sex of the baby do not show any association to the time of birth. The conclusion of the particular study is:

“Human gestational length varies considerably even when measured exactly (from ovulation). An individual woman’s deliveries tend to occur at similar gestational ages. Events in the first 2 weeks after conception are predictive of subsequent pregnancy length, and may suggest pathways underlying the timing of delivery.” View it here

If this leaves you feeling confused, here’s my take-away. The length of a healthy, normal pregnancy is extremely variable. Your due date may be a useful piece of information for you, but it may lead to some stressful interactions with friends and family toward the end of your pregnancy. My advice is to pick a less specific estimation of when your baby will come, maybe cite your most likely birth month when asked about your due date. You can even go less specific than that and tell folks the season that your baby is due. For example, my son was due August 18th, but I often told people that he was due in August or late summer.

My hope is that this can help folks avoid the stress of a barrage of phone calls, texts, and Facebook messages that I see starting to pile up for my clients after 40 weeks. Pregnancy and birth can be stressful, so lets do our best to avoid unnecessary anxiety. If someone you love is pregnant, trust that they will announce their birth when they are ready and resist the temptation to call/text/message unless you’re offering support as the pregnancy continues. If you are pregnant, consider going offline after 40 weeks. Take some time to yourself and your close family to enjoy the last portion of your pregnancy and the ability of your body to grow an entirely new human.

One Comment on “The Low Down on Due Dates

  1. If one baby has been born a natural birth, the second child is very, very likely to birth at the exact same phase of the Moon. It is known in some astrological circles as the mother’s birthing time.

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