The Mysterious Deteriorating Placenta

I once heard an OB say “Babies are like library books. They have a due date, and for every day past the due date, there is a price to pay. And it’s the baby that pays the price!” He was explaining why he has an absolute policy that all of his patients MUST be induced on the day before their due date. But his approach doesn’t jive with current research, and doesn’t take the risks of routine induction into account. (That OB is now retired.) But is the concern about going past the “due date” real?

First, let’s talk about due dates. The 40 week Naegle’s rule that is most often used to calculate due dates isn’t evidence based at all, and might be underestimating the normal length of pregnancy. Rebecca Dekker of Evidence based birth has a great explanation of that in her article on due dates, so I’ll just send you there to read the details.

It’s also important to remember that on your due date, you’re not post term. You’re not even post term the day after your due date. Or the date after that! You are not post term until a full 42 weeks!

Now that we’ve got that aspect out of the way, let’s be clear that the rates of stillbirth do go up after the due date, but the *reasons* for that are not usually spelled out in those studies. The two most common reasons I’ve found mentioned in non-scientific articles for the increase in post term still birth are meconium aspiration and “placental disease”. It’s important to note that placental disease is a broad category that includes preeclampsia, not just “deteriorating placenta”.

Closeup of placenta with calcification
Closeup of the maternal side of a placenta with calcification.
Photo by Robin Baker, used with permission.

Most scientific articles on the causes of stillbirth don’t separate the stats on stillbirths past the due date from all stillbirths. Most lump all stillbirths after 20 or 28 weeks together. I was only able to find one study that broke it down to 37+ weeks, which still is a wide range that crosses the due date. So there’s just not much out there clarifying what might be happening after the due date to explain the uptick in stillbirth rates.

Calcification (small white spots of deposited calcium) is something that is commonly pointed out on placentas as a sign that it was “aging” and not functioning well. However, the research is far from clear on what, if anything, it means to have visible calcification in a placenta.

And finally, I want to address a couple of myths about the placenta:

Myth: The placenta is a filter that gets clogged – The placenta is not a filter so much as a transfer station. Waste from the baby passes through into the parent’s bloodstream, where their kidneys and/or liver get rid of it.

Myth: The placenta suddenly stops working at 42 weeks – It’s not usually a sudden and dramatic shutoff, but rather a gradual decline. This is why it’s important to be aware of your baby’s movements and act on any concern you have about a decline in your baby’s activity level. One theory I commonly read when researching this article is that a lower level of amniotic fluid (which is common at term) increases pressure on the placenta and cord, decreasing blood flow and generally making the whole system less efficient.

The truth is, this is an area of human physiology we know very little about. It’s so rare it’s hard to get large enough numbers to do research studies (which is a GOOD thing!) but that does make it difficult to make evidence based decisions. A recent large scale review of the evidence by scientists with the Cochrane organizations showed that the best we can say is that inducing labor after the due date does have a small decrease in the stillbirth rates, but that the timing on when that should be offered is very unclear.

Further Reading: