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So how is a due date calculated? One way to calculate your EDD (assuming you are fairly regular at a 28 day cycle with ovulation on day 14) is called the Naegele Rule. Start with the first day of your last menstrual period (LMP) add seven days, then add nine months. For example, if your last period started on June 10, 2017, you would add seven days, June 17, 2017 then add nine months, March 17, 2018. But remember, this date is just an estimation as everyone's body's are unique and not many women have the regular cycle with ovulation on day 14 as this method suggests. It is also important to know that this formula is not based on current evidence, but rather on studies done in 1744 and in the 1800's.
Another common way to calculate a due date is through an ultrasound. An ultrasound early in pregnancy has been shown to be more accurate than the Naegele rule, especially in women who don't have regular cycles. Based on this study, finding out gestational age by ultrasound was most accurate between 11-14 weeks with accuracy dropping by 20 weeks. Women who had an ultrasound between 11-14 weeks had delivered their baby ±11 days from the EDD, the most accurate so far.
A more recent study has shown that EDD's shouldn't be calculated at 40 weeks exactly, but rather at 40 weeks 3 days (for second time mothers) to 40 weeks 5 days (for first time mothers). An even newer study looked at 113 healthy women and measured their hormones daily in order to know the exact days the women ovulated, conceived and when their pregnancies implanted. This study found a median time from ovulation to birth of 268 days or 38 weeks, 2 days and the median time from the first day of the last menstrual period to birth was 285 days or 40 weeks, 5 days.
But what do all these weeks and days mean? Let's look at this definition from Evidence Based Birth of full term to understand what weeks are currently considered full term.
"For many years, a baby was defined as being born at “term” if it was born between 37 weeks 0 days and 41 weeks 6 days. Anything before that 5-week period was considered “preterm,” and anything after those five weeks was “post-term. Over time, though, research began to show that health problems were more common at certain points during this 5-week “term” period. In particular, newborns are more likely to die (although the overall risk was still very low) if they are born before 39 weeks, or after 41 weeks. The chance of a newborn having problems is lowest if he or she is born between 39 weeks and 0 days and 40 weeks and 6 days (Spong 2013). In 2012, a group of experts came together to define “term” pregnancy. Based on their review of the research evidence, they broke the 5-week term period into separate groups (Spong 2013)
“Early term” babies are born between 37 weeks 0 days and 38 weeks 6 daysLike EBB says, there is a higher (but still very low) risk when giving birth outside of the "full term" period, which is why OB's often encourage inductions between 38-40 weeks. However, inductions come with their own set of risks and EDD are after all, an estimation and not an exact science.
“Full term” babies are born between 39 weeks 0 days and 40 weeks 6 days.
“Late term” babies are born between 41 weeks 0 days and 41 weeks 6 days
“Post term” babies are born at 42 weeks and 0 days or later"
Inductions can be done in a few different ways and there is no way to naturally induce labor. The most popular way labor is induced in a hospital setting is through synthetic Oxytocin, also called Pitocin. This disrupts your body's natural hormone cycle, causing you to stop producing oxytocin (the love and bonding hormone) and endorphin's (your body's natural pain killer), makes contractions stronger which makes labor more painful and difficult and to manage and causes the uterine muscles to never fully relax between contractions. Sweeping or stripping of the membrane is another way you may be induced. This may cause a higher risk of premature breaking of your water.
Of course there are legitimate medical reasons for inductions in higher risk births where getting baby out quickly is important for health and safety reasons, but for low risk births there is rarely a true medical need for inducing and it most often occurs just because mom has gone past her estimated due date. When induced there is usually a cascading waterfall of interventions to follow including increased risk of cesarean in addition to a baby that may be significantly less mature than if labor were allowed to start on it's own. If you have a midwife or obstetrician who is considering induction because you are past your EDD, ask them about waiting until labor spontaneously begins on it's own. To read more about this, check out The Lamaze 6 Health Birth Practice's, Practice #1- Letting Labor Begin on It's Own.
In conclusion, figuring out the day of your little one's arrival is not an exact science and is hard to predict accurately. Every woman's menstrual cycle is different, and every pregnancy is different, calculating an EDD is an educated guess. Focusing on that date as "the day" will most likely cause unnecessary stress and lead to induction and further interventions. Instead focus on the fact that your baby is using these last days to mature, develop and prepare for life outside of the womb. If you have any further questions or concerns be sure to discuss this with your care provider or doula and read up on the sources below.
Sources and for more information: