Not all popcorn kernels pop at the same time.
The truth is, we can’t tell you when your baby will smile, or crawl, or say “Mama,” and we can’t tell you when your baby will be ready to be born. Everyone is different. Not all the popcorn kernels pop at the same time, and though we can give you an ESTIMATED due date (EDD), we can not tell you when your baby’s development is ideal for birth. Only your baby can tell you that.
The science behind days of pregnancy.
The estimated due date is calculated to be 280 days from the date of your last menstrual period. Doctors and midwives in the U.S. use Naegele’s Rule. Dr. Franz Naegele practiced in the 19th century Germany and noticed that pregnancies in his practice averaged 266 days. That’s 266 days from ovulation, assuming that all women have 28 day cycles and all women ovulate on day 14. We all know cycles vary, even in the same woman, and science has shown that women can ovulate anywhere between days 7-30, sometimes even later.
Let’s imagine you came off the pill right before you got pregnant. That often pushes your ovulation back- up to day 30 or even later. Your EDD will be calculated assuming you ovulated (and therefore got pregnant) on day 14. That’s 16 days off. Meaning your baby isn’t actually ‘due’ until you are 42 weeks and 2 days.
Mamas of color and Mamas blanche.
So isn’t there better science for this? Something since 1820? Yes there is. One such study from 1990 found that mamas of color or who have previous pregnancies average 269 days of pregnancy (note this is different than Naegele’s 280 days from the first day of your last period). White, first time moms averaged 274 days of pregnancy. The study using modern women and modern scientific processes shows we average an extra 3-8 days. But it hasn’t changed obstetrical practice. This may be in part because there have also been other studies that support the continued use of 280 days from last menstrual period. But many women don’t fit that mold.
So, if you’re a first-time, white mama who knows her conception date exactly, you could need an extra 8 days tacked on to your EDD. (Just remember, when figuring conception date, that sperm can hang around waiting for an egg up to five days after sex. You don’t always conceive on the day you do it, and might need to add more days!)
Babies don’t conform past 10 weeks.
What about ultrasound? Embryos develop at the same rate from conception to about 10 weeks (using your handly Naegele EDD). After that, your baby will grow and develop at his own speed for the rest of his life. Ultrasounds that look at development usually aren’t done until about 20 weeks, and then differ by the quality of the equipment and the skill of the clinician. The later the ultrasound, the less reliable they are for estimating development and size.
With very few exceptions, your baby will be born when it is ready. Your baby will come on his birthday. He will pick it. It can help to have a due month rather than a due date. Try not to fixate on the date given to you by Franz in 1820. You will most probably have your baby within a week either side of this date, it’s true. But if you are 41 weeks pregnant it does not mean your baby is late. It means she likes being in there and isn’t ready yet.
How to let your baby pick her birthday.
Unfortunately, it’s a luxury for a baby to pick her own birthday in much of modern maternity practice. When calculating due dates, you can do your part by making sure your care provider knows if you have long cycles, and by being relaxed about your estimated due time. When people ask “when are you due?” you can just say “March” or “late June,” or just tack on a few extra weeks so you get fewer “you’re still pregnant?!” greetings.
Also remember that your body needs oxytocin in large quantities to go into labor. So if you are highly anxious or upset about your baby’s due date nearing or passing, you will be producing more stress hormones than anything else. No matter when you’re ‘due’ or when other people think you are due, just relax, mama. Baby’s got this.