How is the "estimated due date" calculated? Can it be the reason there are so many babies coming "late"?
The moment you start sharing that you're pregnant, there's a single question everyone will want the answer to..."when are you due?"
There are TONS of apps you can use and searching the internet for information on how to calculate your due date will lead you to Naegele's Rule. All the due date apps use Naegele's Rule to create the due date. This rule requires you to know the first day of your Last (NORMAL) Menstrual Period (LMP). Then has you subtract 3 months and add 7 days to discover your estimated due date (EDD).
VOILA! The date your baby will be born.
BUT WAIT! How accurate is this date?
And....who in the world was Naegele?
How did he create his equation?
These are all GREAT questions!
Dr. Franz Karl Naegele, was an Obstetrician in the early 1800s. Back in those days, it was common to not know the date to expect your baby to be born. Everyone knew what a pregnant woman looked like when she was "close" to giving birth AND everyone also knew that the baby would come...eventually.
Dr. Naegele decided that it would be so much easier if there was a way to know when to expect the baby so came. This prompted him to come up with the above computation that could be used to roughly calculate when a woman would have her baby.
He "declared" (eye roll) that pregnancy lasts 10 lunar months, counting from the first day of the last menstrual period. I'm not even going to go into all the things that come along with anyone "declaring" how long a natural bodily function takes to complete, okay?
He also assumed that all women had 28-day menstrual cycles. Now, in his day (1806), this was most likely true. It is, however, no longer true for women today (2022). If you're charting your cycles, you know how long they are and you just might not have a 28-day cycle. Sadly, not all women chart their cycles. (Remember this point because I'll come back to it...)
A lunar month is 28-days. According to Dr. Naegele, the length of human gestation is 280 days (10 lunar months x 28 days = 280 days).
EXAMPLE: LMP = 12-01-2021 - 3 mos + 7 days gives an estimated due date of 09/07/2022.
Easy peasy, right?
Naegele's Rule - when starting with the first day of a woman's last < normal > menstrual period - does not take into account the number of days the woman is not pregnant.
Nor does it take into account the length of a woman's cycle that goes
past 28-days. It also doesn't account for the fact that fertilization
may not happen immediately after ovulation.
If you are charting your cycle, you will know (1) how long your cycle is, (2) whether or not it varies from month to month, (3) what day of your cycle you ovulate, and (4) the most likely date of conception.
That's not all...
In the late 1990s, a study was done by Mittendorf et al. where they measured the median duration of pregnancy. They found that healthy, white, private-care, first time moms (primiparous) with well-established due dates averaged 288 days and multiparas (pregnant with 2nd+ baby) averaged 283 days. This study also showed differences in duration of pregnancy according to ethnicity. For example, black women averaged 8.5 days fewer than white women of similar socioeconomic status.
Let's go back to our example from above.
LMP = 12/01/2021 - 3 months + 7 days = 09/07/2022. If this is the first pregnancy for this woman and she meets all the above mentioned criteria (healthy, white, with private care), then we need to add 1 week. This places her estimated due date at 09/14/2022. This still doesn't account for the other variables mentioned:
Carol Wood, a Yale Nurse Midwife Professor, came up with a method of calculation that takes into account individual variations in length of menstrual cycle as well as the effect of a woman having had previous pregnancies. With her method, you:
(1) Add 1 year to the first day of your last < normal > menstrual period, then
(2) Then add or subtract the number of days your cycle varies from 28-days.
Let's see what happens when we use this method with our example of LMP = 12/01/2021
LMP = 12/01/2021 + 12 months - 2 months = 10/02/2022 - 14 days (remember this is her first pregnancy) = 09/18/2022
That adds 11 days to the EDD we got when we used Naegele's Rule. Let's say this woman knows she has a 28-day cycle. So we're good, right? Well... the Wood's Method doesn't take the Mittendorf study into account. Since it's this woman's first pregnancy, we could add another 7 days to give her the EDD of 09/25/2022.
NOW is it accurate?
If this woman were my client, I'd give her the estimated due month of 09/07/2022 to 10/09/2022.
Why? There's much more to estimating the birth of a tiny human than just counting days on a calendar. Remember that Dr. Naegele "declared" that pregnancy lasts 10 lunar months.
Was he right? I have no idea. What I do know is that with the "estimated" due date, fundal height measurement, palpating the belly, and tuning in to when the mom first starts to feel her baby's movements, we can widdle things down a bit.
Studies have shown that ultrasound-determined EDDs are not accurate. As a matter of fact, they can be off +/- 5 to 22 days. Estimated due dates based off your the first day of your last < normal > menstrual period should never be changed based solely on sonogram/ultrasound dating. In most cases, you know when your last < normal > menstrual period was, especially if you're charting your cycles. Even if you're not sure what your LMP is, fundal height measurement, belly palpation, and quickening (when you feel the baby move for the first time) will give you a more accurate EDD than using sonogram technology.
12/01/2021 is the actual LMP of one of my clients. She gave birth 10/06/2022.
According to Naegele's Rule, she was 44 weeks + 1 day gestation. Using Wood's Method and adding 1 week according the Mittendorf study, put her at 41 weeks + 4 days gestation when her little one was actually born.
As we agreed to make this change, we went through her chart. Her fundal height measurements along with what I felt as I palpated her belly, were inline with her new EDD of 09/25/2022.
As I completed the visual portion of the newborn exam, it was determined that she was around 42 weeks gestational age at birth.
It's worth taking the time to discover your estimated due MONTH instead of setting your sights on a single date.
Please share your personal journey with estimated due dates in the comment section below!