When a woman finds out she’s pregnant, one of the first things she wants to know is her is due date. There are formulas for figuring it out manually, but most use a little “wheel” (like the doctors) or an online “quiz” that will tell you your baby’s due date. Many first-time moms circle this due date on the calendar, as well as the date three weeks prior, when they reach 37 weeks or “full term.” These are major milestones for them!
However, due dates aren’t all they’re cracked up to be, for several reasons. And since due dates are (unfortunately) used to make serious decisions about things like labor inductions and scheduled c-sections (which we’ll talk about next week), it’s important to understand them.
What’s in a Due Date?
A due date is typically calculated from your last menstrual period. It is based on the idea that pregnancy lasts approximately 280 days from this point, or about 40 weeks or 9 months. An OB noticed that many women seemed to have their babies around this time, and came up with a formula for finding out approximately when to expect babies to be born. As medical science advanced, they noticed that babies born after around 37 weeks were typically fully developed and did not have any major lung or other body system issues. This became considered “full term.”
When you find out you are pregnant, your doctor or midwife will typically use a little wheel based on the averages above and your last period to come up with an approximate due date for you. But don’t be so quick to circle it on the calendar! A lot of factors can throw off your due date, sometimes by weeks.
Why Aren’t Due Dates (Necessarily) Accurate?
The idea that a due date can be derived from your last period assumes that you (and all women) have regular 28-day cycles, and that ovulation occurs around day 14. This, as we know, is not true for all women. Since the due date is calculated from your period, which is at least two weeks prior to your conception date, an irregular cycle can really throw a wrench in the mix.
These are some factors that can throw off your due date:
Ovulation typically occurs about 14 days before the start of your next period. If you have a 28-day cycle, that’s also roughly 14 days after your last one. But if you have 35-day cycles, or 60-day cycles (which some women do), or 18-day cycles — any variation — your ovulation won’t have occurred “right in the middle.” This means your baby could be due a week or two (or more) earlier or later than your calculated due date! If you have irregular cycles, it is very important to let your care provider know so they can attempt to adjust for this.
You may know your baby’s conception date. You may not. It may or may not line up with when the calendar says you “should” have ovulated. This usually will only throw things off by a day or two, but it’s still important. If you’re familiar with your body and know what ovulation ‘feels’ like, or have a BBT chart, share your conception date with your doctor for more accurate dating.
Rate of Development
All babies develop differently, even before they’re born. Some babies may only need 38 weeks. Some need 46! It is impossible to tell how long your baby will really need to be inside before being developed enough to be born. Due dates are mere averages and guesses; they are not (and cannot) be exact.
Many providers will send women to get ultrasounds in their first trimesters to “date” the pregnancy. While this is more accurate in the first 12 weeks than beyond (because the development of the baby from a ball of cells to a tiny “baby” appearance is more pronounced and obvious), it is still not a guarantee. This date can still be off by a few days or a couple of weeks. An ultrasound dated my first being due a few days later than she actually was (thankfully we didn’t induce, but she wasn’t as “early” as they thought — she was actually 4 days early. I knew my conception date for certain and they used the ultrasound over my knowledge).
Why Does This Matter?
Unfortunately, since most modern OBs believe strongly in “the due date,” a lot of decisions can be made or strongly suggested about your care based on the due date. When I went in for my 39-week appointment with my first baby, everything was looking great, baby and I were healthy, but the doctor said, “When you come back next week, we’ll discuss induction.” Just because I had reached my due date! I went home crushed. (I had the baby 3 days later on my own.)
Recommending induction, or “allowing” induction (if a woman is ‘tired of being pregnant‘) often happens based on the due date. Some providers will schedule inductions any time past 37 weeks, and certainly past 39. They rarely discuss the risks of this, either. Most providers want to discuss induction by 40 – 41 weeks, and very few will “let” their clients go past 41 weeks.
The average first pregnancy for a healthy woman who is not induced is 41 weeks. Average. There will be women who naturally deliver at 39, and women who naturally deliver at 43. And that’s all assuming that their dates are really correct in the first place!
When a baby is ready to be born is so subjective. How many weeks the baby is does not tell you how big the baby is, how developed the lungs are, how developed the sucking reflex is, etc. It does not tell you if the baby is actually ready to live on the outside.
Informed Consent With Due Dates
More and more doctors and midwives are starting to do away with a “due date model.” Instead, they’re looking at a “due week.” That is, for example, “We think the baby will be born around the second week of March.” (Which is my “due time” with baby #4.) This is a much better model because it allows for flexibility, rather than pinning one’s hopes and decisions on just one date.
Even so, informed consent is important. Unless there is a medical reason (you have high blood pressure, baby is in distress, you have developed pre-eclampsia, your water breaks, etc.), there is no need to suggest interventions up to and including inductions and c-sections based on how long you have been pregnant.
Ask your doctor for evidence-based, individual care. You do not have to consent to any procedure based on the fact that it is the office or doctor’s policy to do something after a woman reaches a certain point in her pregnancy. Be aware of that, and speak up. “As long as my baby and I are doing well, I prefer not to be induced, even if I go post-date.”
Next week we will talk specifically about induction — risks, benefits, situations where you may consider it, and so on.
What do you think about due dates? Do you circle them on your calendar or do you take them with a grain of salt?
Confused about vaccines?
Get our FREE no-nonsense vaccine guide. Answer your questions with rational, fact-based information instead of fear.