Does the Placenta Really Stop Working After 40 Weeks? What the Evidence Actually Says.
- Joy Cassidy
- Aug 12
- 10 min read
If you’re approaching your due date, there’s a fair chance somebody will mention your placenta.
Perhaps you’ve heard:
“Your placenta starts to deteriorate after 40 weeks.”
“Your placenta might stop working.”
“Baby needs to be born before the placenta fails.”
That sounds pretty alarming.
After all, the placenta is your baby’s lifeline during pregnancy. If somebody tells you it could stop working, saying yes to an induction can suddenly feel less like a choice and more like the only sensible thing to do. But, is that actually what happens? The short answer is no, your placenta does not suddenly stop working when you reach 40 weeks.
Your placenta doesn’t know that Tuesday is your due date and promptly decide its work here is done.
That said, placentas do change and age as pregnancy progresses, and research shows that some risks gradually increase as pregnancy continues beyond 40 and particularly 41 weeks.
So this isn’t about dismissing concerns around prolonged pregnancy. It’s about understanding what we actually know, what we don’t know, and what those risks might mean for you.
First things first: your due date isn't an expiry date
Your estimated due date is exactly that: an estimate. Only a small proportion of babies arrive on that exact day. A pregnancy reaching 40 weeks doesn't suddenly become abnormal, and NICE guidance specifically says that women with uncomplicated pregnancies should be given every opportunity to go into labour spontaneously.
In the data used by NICE, a significant proportion of spontaneous labours begin during the 40th and 41st weeks of pregnancy. So being 40+1 or even 41 weeks pregnant does not automatically mean something has gone wrong.
It simply means that, as the pregnancy continues, the balance between waiting for labour to begin and choosing to intervene gradually changes.
What does your placenta actually do?
The placenta is a pretty remarkable temporary organ.
During pregnancy it helps:
transfer oxygen from you to your baby
transfer nutrients to your baby
remove carbon dioxide and waste products
produce hormones needed to maintain pregnancy
regulate aspects of your baby's growth and development.
It develops alongside your baby and is designed to function throughout pregnancy.
But like every biological organ, it isn't completely unchanged from beginning to end.
Does the placenta age?
Yes. Placental ageing is a real biological process.
Researchers examining placentas at different gestations have found evidence of increasing cellular ageing, known as senescence, alongside oxidative stress and other structural and functional changes as pregnancy advances.
Studies of prolonged pregnancies have also identified changes including reduced placental growth, reduced blood vessels within some of the tiny placental structures involved in exchange, and changes in the processes used to transport substances between parent and baby.
So saying:
“Placentas don't age.”
wouldn't be accurate.
But neither would saying:
“Your placenta wears out at 40 weeks.”
There is a very large gap between those two statements.
Placental ageing is not the same as placental failure
This distinction really matters.
Ageing does not mean that an organ suddenly stops functioning. A placenta can show signs of maturation or ageing and still be doing its job perfectly well. Researchers studying prolonged pregnancy suggest that some placental changes may initially be normal adaptations. In some pregnancies, however, ageing processes may eventually contribute to reduced placental function.
And that's really the issue.
There isn't a particular gestational age at which every placenta becomes inadequate. One placenta may continue to function perfectly well while another pregnancy may develop evidence of placental insufficiency earlier. Unfortunately, there is not a simple test that can say:
“Your placenta has exactly five good days left.”
What about a “calcified” or “mature” placenta?
You may also hear somebody describe the placenta as looking “mature” or “calcified” on an ultrasound scan. Calcification means that small calcium deposits can be seen within the placenta. Placental appearance does change as pregnancy advances, and calcification near term can be part of normal placental maturation.
Interestingly, placental appearance on ultrasound is not a straightforward measure of how well the placenta is functioning. Research into placental grading has produced mixed findings, and older studies found that the appearance of a very mature-looking placenta at term could not reliably identify a post-term pregnancy.
Premature calcification - particularly when seen well before term - is a slightly different situation and has been associated in some studies with complications such as fetal growth restriction and pre-eclampsia.
So again, context matters. A placenta looking mature at 40 weeks is not the same thing as being told your placenta is failing.
If the placenta doesn't suddenly stop working, why are we concerned about going overdue?
A very large study of around 15 million pregnancies found that the risk of stillbirth remained low, but increased slightly as pregnancy continued beyond 40 weeks. That doesn't mean we can say the increasing risk is simply caused by the placenta “wearing out” – pregnancy and placental function are much more complicated than that.
The important thing to understand is that the risk does not suddenly become high at 40 or 41 weeks, it gradually increases over time.
What changes after 41 weeks?
Current NICE guidance says that some risks associated with pregnancy continuing beyond 41+0 weeks may increase over time, including:
stillbirth
neonatal death
admission of the baby to neonatal intensive care
caesarean birth.
Notice the wording.
It doesn't say:
“At 41 weeks your placenta stops working.”
It says that risk may increase over time.
That is a much more accurate way of thinking about prolonged pregnancy. There isn't a cliff edge at 40 weeks or 41 weeks. Risk exists on a continuum.
So why is induction often offered around 41 weeks?
Researchers have compared what happens when uncomplicated pregnancies are induced at around 41 weeks with what happens when women wait longer for labour to start on its own.
Overall, the research suggests that induction at 41 weeks may reduce some of the small risks associated with continuing pregnancy, without increasing the chance of having a caesarean birth.
One large trial, involving more than 1,800 women with uncomplicated pregnancies, found slightly fewer serious problems for babies in the group induced at 41 weeks compared with those who waited until 42 weeks.
Other research has found similar results, which is why NICE recommends discussing induction from 41 weeks.
However, it’s important to keep this in perspective: most babies in both groups were born well, and the overall chance of a serious problem remained low. This is why induction is offered as an option, not because something suddenly becomes dangerous at 41 weeks, but because the balance of risk gradually changes as pregnancy continues.
Doesn't induction increase the chance of a caesarean?
This is one of the most common concerns I hear when we talk about induction.
Induction changes the experience of labour. It may involve cervical ripening medication or a balloon catheter, breaking the waters, an oxytocin drip and additional monitoring. NICE also acknowledges that induced labour can be more painful and may affect choices around place of birth and some aspects of labour.
But when researchers specifically compare induction at 41 weeks with waiting until 42 weeks, the best randomised evidence does not show an increased rate of caesarean birth.
The large meta-analysis found caesarean rates of approximately:
10.5% with induction at 41 weeks
and
10.7% with waiting until 42 weeks.
It doesn't mean that induction has no downsides. It means that “induction automatically leads to caesarean” is too simplistic.
Can monitoring tell whether my placenta is still working?
You might reasonably think: “If I don't want an induction, can't we just monitor the baby and make sure the placenta is still working?”
Monitoring can certainly provide useful information. Depending on your gestation and circumstances, this might include checking your baby's heart rate with a CTG and assessing the amount of amniotic fluid by ultrasound.
Monitoring has limitations though and NICE is very clear about this. Monitoring tells healthcare professionals how your baby appears to be at that moment. It cannot reliably predict what will happen hours or days later.
A reassuring CTG today therefore cannot guarantee that everything will remain reassuring tomorrow. That doesn't make monitoring useless, it simply means it provides a snapshot of how your baby is doing at that particular time.
What if I don't want induction at 41 weeks?
An offer of induction is exactly that: an offer.
You can accept it.
You can decline it.
You can ask whether it could happen a little later.
You can ask for more information.
You can change your mind.
NICE specifically states that women can proceed with, delay, decline or stop an induction and that their decision should be respected.
If you decide not to have an induction, your maternity team should discuss your options and make a plan with you.
NICE suggests discussing additional fetal monitoring from 42 weeks for women choosing to continue their pregnancy, while also explaining its limitations.
What happens if you're overdue in Scotland?
If you're pregnant in Edinburgh, West Lothian, Falkirk, Linlithgow or elsewhere in Scotland, the exact arrangements will depend on your maternity service and your individual pregnancy.
NHS Inform says that if you've had a straightforward pregnancy and are overdue, induction is usually offered between 41 and 42 weeks.
Locally, NHS Lothian says induction may be offered when pregnancy has continued beyond 41 weeks. Their induction information also makes it very clear that the decision to have an induction or to wait is yours and that your choice will be respected.
If you decide not to be induced when it is recommended, NHS Lothian says you will be offered monitoring to check how you and your baby are. Importantly, they also explain that this monitoring tells you how your baby is doing at that moment, which fits with what we discussed earlier about monitoring being a snapshot rather than a guarantee of what will happen later.
For families receiving care through NHS Forth Valley, their information says that following a straightforward pregnancy, induction is offered between 10 and 14 days after the estimated due date. They describe the placenta after this point as something that “may become less efficient”, rather than saying that it suddenly stops working.
Questions to ask when induction is offered because you're overdue
If you're sitting in an appointment trying to make this decision, it can be difficult to think of questions on the spot.
You might want to ask:
Why are you recommending induction at this particular point in my pregnancy?
Are there any factors in my pregnancy that change my individual risk, or is this being offered routinely because of my gestation?
What are the benefits of induction now compared with waiting another few days?
Can you explain the risks in absolute numbers rather than relative risk?
What would my options be if I decided to wait?
What monitoring would be offered if I continued the pregnancy, and what can that monitoring actually tell us?
Would waiting change any of my birth options?
What would the induction process involve at this hospital?
Can I have some time to think about it before deciding?
The point of informed decision-making is that somebody explains the information well enough for you to decide what feels right for your circumstances.
FAQ: Placenta ageing, overdue pregnancy and induction
Does the placenta stop working at 40 weeks?
No. There is no evidence that every placenta suddenly stops functioning at 40 weeks.
Placental ageing is a genuine biological process, but it happens gradually and varies between pregnancies.
Does the placenta start deteriorating after your due date?
The word “deteriorating” makes this sound much more sudden than it really is. Research shows structural and cellular changes in placentas as pregnancy advances, and reduced placental function is one possible explanation for the increasing risks seen in prolonged pregnancy.
However, reaching your due date does not mean your placenta has become inadequate.
Can a placenta work after 42 weeks?
Yes, some placentas continue to support babies beyond 42 weeks. The problem is that we can't accurately predict which pregnancies will remain completely uncomplicated and which will develop problems. Population data show increasing risk as pregnancy advances, which is why induction is generally offered before or around this point.
Does placental calcification mean my placenta is failing?
Placental calcification simply means that small calcium deposits have formed within the placenta. These deposits are thought to develop mainly in areas where placental cells and tissue are naturally ageing or breaking down, allowing calcium and phosphate to settle there. This becomes more common as pregnancy progresses and, particularly near term, can be part of normal placental maturation. Seeing calcification on a scan does not automatically mean the placenta is failing. What matters is the wider picture, including your baby’s growth, movements and any other scan findings.
Calcification occurring unusually early in pregnancy may be more significant and has been associated with conditions including fetal growth restriction and pre-eclampsia, so it needs to be interpreted alongside the rest of your pregnancy.
Is induction compulsory at 41 weeks?
No.
Induction requires your consent.
“NICE guidance recommends discussing induction from 41 weeks because evidence suggests that it may reduce some of the risks associated with continuing pregnancy.”
You still have the right to discuss the alternatives and make your own decision.
Is it dangerous to go past your due date?
Simply reaching 40 weeks does not suddenly make pregnancy dangerous. However, research involving millions of pregnancies shows that the absolute risk of stillbirth and some other adverse outcomes gradually increases as gestation advances.
The individual risk is still small, but it isn't zero.
Can a CTG prove my placenta is working properly?
A reassuring CTG gives useful information about your baby's wellbeing at that moment.
It cannot prove that the placenta will continue to function normally over the following hours or days.
The bottom line
So, does your placenta stop working once you reach 40 weeks? No.
Your due date isn't your placenta's expiry date, but the other extreme – saying there is no reason at all to be concerned about prolonged pregnancy – isn't supported by the evidence either.
Placentas do age. Some placentas may become less efficient as pregnancy continues and across large populations, the chance of some serious outcomes gradually increases with advancing gestation.
If you're preparing for birth in Linlithgow, West Lothian, Edinburgh, Falkirk or the wider Forth Valley, this is exactly the sort of decision-making we explore in my antenatal courses. Not because there's one “right” answer to every situation, but because birth is much easier to navigate when you understand your options, know what questions to ask and don't have to make decisions from a place of fear.
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