Study: Risk of Stillbirth Week by Week (late third trimester)

Not so scary if you go post dates after all.........

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The risk of unexplained stillbirth at:

35 weeks is 1:500
36 weeks is 1:556
37 weeks is 1:645
38 weeks is 1:730
39 weeks is 1:840
40 weeks is 1:926
41 weeks is 1:826
42 weeks is 1:769
43 weeks is 1:633

BMJ. 1999 July 31; 319(7205): 287–288.
PMCID: PMC28178
Copyright © 1999, British Medical Journal

Prospective risk of unexplained stillbirth in singleton pregnancies at term: population based analysis

Christina S Cotzias, research registrar,a Sara Paterson-Brown, consultant,b and Nicholas M Fisk, professora
aInstitute of Obstetrics and Gynaecology, Imperial College School of Medicine, Queen Charlotte’s and Chelsea Hospital, London W6 0XG, bQueen Charlotte’s and Chelsea Hospital

Contributors: CSC refined the methodology, collected and analysed the data, and drafted the paper. SP-B contributed to the study design, analysis, and paper drafts. NMF had the original idea for the study and contributed to drafting and revising the paper. CSC is guarantor for the study.

Correspondence to: Sara Paterson-Brown s.paterson-brown@rpms.ac.uk
Accepted February 25, 1999.

This article has been cited by other articles in PMC.

Unexpected late fetal death is tragic but not uncommon, most such fetal deaths being unexplained. Although five times more common than sudden infant death,1 they have attracted scant public attention.

Delivery is recommended when the risks to the fetus in utero are greater than those to the baby after birth; in high risk pregnancies this is generally believed to be around 38 weeks. The risk of unexplained stillbirth near term is, however, relevant to all pregnancies. Current numerical estimates do not detail risk by gestation,1 and the few studies that have done so are no longer applicable in the United Kingdom in the late 1990s. Yudkin et al calculated the total risk of stillbirth by gestation using population data that are currently over 15 years old,2 while Feldman calculated a prospective risk using data from a New York City population, including multiple pregnancies and a high proportion of women with no antenatal care.3 We calculated prospective risks of unexplained stillbirth by gestation in singleton pregnancies near term.

Methods and results

We reviewed published data on 171527 births in the North East Thames region in 1989-914 and derived the number of ongoing pregnancies and stillbirths at or beyond each gestational week from 35 to 43 weeks. The prospective stillbirth rate per 1000 ongoing pregnancies was calculated as the number of stillbirths at or beyond week n divided by the number of pregnancies at or beyond week n multiplied by 1000, where n is the week of gestation from 35 to 43 weeks.

As the original dataset included all stillbirths (explained and unexplained and those in multiple pregnancies), we applied correction factors to derive gestation specific risks of unexplained stillbirth in singleton pregnancies near term as follows. We used data from the Office for National Statistics for 1994 to estimate the proportion of all births (live births and stillbirths) that were singleton (650826/659545=0.9868) and the proportion of overall stillbirths that were in singleton pregnancies (3465/3813=0.9087). We used data from the 1994 confidential enquiry into stillbirths and deaths in infancy1 to estimate the proportion of total stillbirths of fetuses >2500 g that were unexplained (833/1137=0.7326).

The table shows the risk of stillbirth in ongoing pregnancies. At or beyond 38 weeks one in 730 singleton pregnancies were complicated by an unexplained stillbirth at term and one in 529 by stillbirth of any cause. Stillbirths of any cause may be more relevant because all stillbirths beyond 38 weeks are arguably unexpected since fetuses with recognised risk factors have usually been delivered by this time.

Comment

We acknowledge that the risks we report are approximations, being derived from three sources of data, but they provide the first quantifiable estimate of risk in continuing singleton pregnancies near term. This information is relevant to modern obstetric practice, where women want to be informed and have high expectations about the safety of their unborn child. Most women would want a caesarean section if the risk of fetal death or damage to their child exceeded one in 4000.5 Our calculations show that the risk of stillbirth at term is five to eight times higher than this.

Interestingly, at 38 weeks the risks of stillbirth near term exceed those at 42 weeks, when delivery is usually recommended. Delivering women routinely at 38 weeks would lead to a high incidence of caesarean section with its attendant risks, either primarily or from failed induction, in addition to a small risk of iatrogenic neonatal respiratory morbidity.

Antepartum stillbirth is a major public health problem, accounting for a greater contribution to perinatal mortality than either deaths as a consequence of prematurity or the sudden infant death syndrome.1 Research into the underlying mechanisms and aetiological factors of this problem to identify pregnancies at risk must remain a prerequisite for any selective strategy to prevent these deaths.

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That's so interesting..
so, if you get past 42 weeks the risk actually declines again

So there must be other studies saying different things in the past, because the whole "increased risk after 42 weeks" belief is very prevalent.

Maybe the risk of stillbirth is lower, but I thought the danger was related to a deteriorating placenta and brain damage. Generally delivering before 42 weeks is normal. It is the 43rd week when the risks become greater (both for stillbirth and brain damage). This issue is close to my heart as my boys were born 40 +11 days and 40+ 10 days and I had to fight being induced both times!

The policy of inducing at 41 or 41 plus3 weeks came from data from a 1963 study suggesting that the risk of stillbirth doubled from 1% to 2% between 40 and 42 weeks. This is what medical practice and advice is based on, but clearly the information is outdated.

The study above is not the only one to show that birth at 42 and even 43 weeks actually carries less risk than at 37 or 38 weeks (and we're told that 37 weeks is full term! It is, but only for some babies). There are also studies showing that the average length of pregnancy, especially for first timers, is at least 8 days longer thsn 40 weeks (eg Mittendorf et al, 1990). The 40 week thing is based on miscalculations in the 1700's from a bible passage, believe it or not!!

Wish I'd known all this before I agreed to an induction at 42 weeks, to deliver, via ventouse after a harrowing and agonising Sintocinon birth, my perfect but tiny and vernix covered daughter, to hear the midwife say, 'oh, she looks a bit premmie! Looks like she could have done with a few more weeks in there".....!!!!!!!!

Thanks, Kelly, for bringing this study to light! :D

Some interesting thoughts, thanks.
My OB said he was concerned about going past 42 weeks as the risk of stillbirth increased significantly. I had also heard this from midwives.
But, I also read something referencing a study that the average gestation for first-time mums was about 41+3 or something like that. (maybe it was the same study you refer to finsmum)

I wonder how much of this is psychological - when a baby is stillborn at 42 weeks, the natural reaction is to think "Well if we'd delivered earlier, this wouldn't have happened"...

Thats been said by various studies tho, from: Naturally Inducing Labour - Natural Induction Methods

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According to a study on ‘The length of uncomplicated human gestation’ by the Department of Epidemiology, Harvard School of Public Health in Boston, Massachusetts, they found:

“For primiparas (first child), the median duration of gestation from assumed ovulation to delivery was 274 days, significantly longer than the predicted 266 days. For multiparas (more than one child), the median duration of pregnancy was 269 days, also significantly longer than the prediction. Moreover, the median length of pregnancy in primiparas proved to be significantly longer than that for multiparas”

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Or... just thinking out loud here...
Does the risk of stillbirth only appear to decrease because so few babies are 'allowed' to go so far? Obviously induction of post-date babies is quite common, I can't help but think perhaps the studies are flawed because the vast majority of doctors do suggest and the (usually sick of being pregnant by that stage) mother accepts an induction within a week or so of going past the EDD? So if nobody was induced for going post-dates, would the rate of stillbirths in 41w+ babies then increase because the risk IS there but hardly anybody actually ever gets to 42+ weeks to find out, kwim?

ETA: From my own experience, I would be very reluctant to allow myself to go past 41w. DD1 was induced at 39+5 and her placenta was gorgeous and healthy, but DD2 went to 40+7 and her placenta was pretty much dead - nothing had been picked up but I knew the minute I saw it, and the midwife confirmed, that if I hadn't been booked in for an induction for the very next day (I went into labour spontaneously), chances are she would either have not made it another 24 hours, or would have come very, very, very close because the placenta was so badly deteriorated. Just my experience, obviously, but something I'd be very wary of if I were planning any more babies.

Given that it's a ratio type of result and the way they worded how they worked out the figures, I would assume it would be out of babies born at that week. So if 500 babies were born at 42 weeks, then they would work out a ratio dependent on the stillbirths x:500

I understand what you mean Glamourcide. For me, my placenta was absolutely wonderful at 42 weeks, so I'd feel fine going longer now.

Marcellus, I know exactly what you mean. My placenta looked absolutely perfect and healthy, as did my baby that was covered in vernix at 43w+5d

edaj8 - how did you get to go so long? Seems like pretty much everywhere they're nervous as hell after 42 weeks.

Homebirth with a trusting midwife (who was rapidly running out of confidence in my baby to leave of her own accord lol)

Ah!
Yeah, 43+5 would test anyone I think.

but then despite my dates saying that I was 43 + 5, both of my pregnancies appear to have been misdated by approximately 2 weeks, I delivered a 37 +5 weeker who was fairly clearly not full term, he had little sucking reflex, was unable to maintain his temperature, and generally had a few minor health issues going on and then our 43 +5 baby whose palcenta looked great, her skin was not peeling, her nails were not long, she was covered in vernix. I think that more likely I delivered a 35 + 5 and a 41+5 (or thereabouts), that my body carries for a bit longer naturally, or some other quite plausible reason for not fitting into the 40 weeks exactly mould. Admittedly after a full 41 weeks wer eup we started doing lots of things to try and help bubby out, acupuncture, massage etc. We also did a membrane sweep at 42 weeks...which clearly didn't do much since i went another 10 days lol.

All of that said and done I would be VERY reluctant to seek medical induction prior to a FULL 42 weeks.

Interesting material. But if I am reading correctly, this is only from "unexplained" stillbirths. Therefore wouldn't "explained" stillbirths, particularly with reference to placental deterioration/failure still be higher post dates.

I also think that the results of any study into normal gestation times is bound to be skewed now by our haste to intervene. Most women who go past even their estimated 41 weeks will be induced and so will never know how long they may have gestated or how healthy thier baby may have been if left to come in their own time. There is of course a rick of placental degradation, but I do also know of women who have had badly degraded placentas way before 40 weeks. I read another study by the Mayo clinic I think showing that the safest model was expectant management, ie monitoring and otherwise leaving nature to do her thing. I totally understand how after a frightening experience like that you would feel nervous about future longer pregnancies, just as after my experience I would never allow induction without a clear medical reason. It's a balancing act, there are risks on either side; as they say, birth is as safe as life gets...

Hi ladies,

I just wanted to clarify something - someone had said earlier that the risk of stillbirth declines after 42 weeks. Actually, this is not true - the risk of stillbirth actually INCREASES again after 40 weeks. I've added some comments to the figures to help with the interpretation. Basically, the lower the ratio number, the higher the risk (and vice versa). So the risk is actually highest at 35 weeks, and decreases until 40 weeks, at which point it begins to increase again. So this is, in fact, another study pointing to the increases in risk that come with going past 40 weeks.

Hope that helps!


35 weeks is 1:500 *** highest risk of stillbirth
36 weeks is 1:556 * risk begins to decrease
37 weeks is 1:645
38 weeks is 1:730
39 weeks is 1:840
40 weeks is 1:926 *** lowest risk of stillbirth
41 weeks is 1:826 * risk begins to increase again
42 weeks is 1:769
43 weeks is 1:633 *risk is about the same as at 37 weeks