Caesarean births 'affecting human evolution

[LaCroix]

http://www.bbc.com/news/science-environment-38210837

Caesarean births 'affecting human evolution'
By Helen Briggs BBC News

5 hours ago
From the section Science & Environment 403 comments

Share
Image copyright Thinkstock
Image caption In the UK, about one in four babies is born by Caesarean

The regular use of Caesarean sections is having an impact on human evolution, say scientists.

More mothers now need surgery to deliver a baby due to their narrow pelvis size, according to a study.

Researchers estimate cases where the baby cannot fit down the birth canal have increased from 30 in 1,000 in the 1960s to 36 in 1,000 births today.

Historically, these genes would not have been passed from mother to child as both would have died in labour.

Researchers in Austria say the trend is likely to continue, but not to the extent that non-surgical births will become obsolete.

Dr Philipp Mitteroecker, of the department of theoretical biology at the University of Vienna, said there was a long standing question in the understanding of human evolution.

"Why is the rate of birth problems, in particular what we call fetopelvic disproportion - basically that the baby doesn't fit through the maternal birth canal - why is this rate so high?" he said.

"Without modern medical intervention such problems often were lethal and this is, from an evolutionary perspective, selection.

"Women with a very narrow pelvis would not have survived birth 100 years ago. They do now and pass on their genes encoding for a narrow pelvis to their daughters."
Opposing forces

It has been a long standing evolutionary question why the human pelvis has not grown wider over the years.

The head of a human baby is large compared with other primates, meaning animals such as chimps can give birth relatively easily.

The researchers devised a mathematical model using data from the World Health Organization and other large birth studies.

They found opposing evolutionary forces in their theoretical study.

One is a trend towards larger newborns, which are more healthy.

However, if they grow too large, they get stuck during labour, which historically would have proved disastrous for mother and baby, and their genes would not be passed on.

"One side of this selective force - namely the trend towards smaller babies - has vanished due to Caesarean sections," said Dr Mitteroecker.

"Our intent is not to criticise medical intervention," he said. "But it's had an evolutionary effect. "
Future trends

The researchers estimated that the global rate of cases where the baby could not fit through the maternal birth canal was 3%, or 30 in 1,000 births.

Over the past 50 or 60 years, this rate has increased to about 3.3-3.6%, so up to 36 in 1,000 births.

That is about a 10-20% increase of the original rate, due to the evolutionary effect.

"The pressing question is what's going to happen in the future?" Dr Mitteroecker said.

"I expect that this evolutionary trend will continue but perhaps only slightly and slowly.

"There are limits to that. So I don't expect that one day the majority of children will have to be born by [Caesarean] sections."

The research is published in the journal, Proceedings of the National Academy of Sciences.

Commenting on the study, Smithsonian paleoanthropologist Dr Briana Pobiner said there are "probably many other biological and cultural issues that factor into the Caesarean sections rate, which varies widely across the developed and developing world".

And Daghni Rajasingam, a consultant obstetrician and a spokesman for the Royal College of Obstetricians, said factors such as diabetes and obesity, are having an impact on the number of Caesarean sections.

"I think what is important to take into the [question of] evolution is that things like diabetes are much more common at a younger age so we see many more women of reproductive age who have diabetes," she said.

"That has consequences as to whether or not they may need a caesarean section.

"In addition, the rates of obesity are increasing so more and more women of reproductive age have a higher body mass index and this again has an impact on caesarean section rates."

Quite interesting, and since such a development has a logarythmic element to it as the trait is handed down, we might see a even bigger change in the next few decades. Evolution continues, even though we think we have uncoupled from it.

[Simon_Jester]

Yeah, but this is a bit like noting that the sickle-cell anemia gene will eventually get bred out of populations when other cures arise for malaria so that malaria resistance is no longer so important that a risk of your children getting sickle-cell anemia is worth it.

It's not that we've uncoupled from evolution, it's that we've uncoupled from evolutionary pressures that used to force specific (and sometimes costly) adaptations.

[LadyTevar]

In my area, Caesarean sections were also on the rise because certain doctors in the area believed their determined Due Date was when a mother would have the child, and if the child wasn't out by the due date, then it was time to cut the baby out.

[Simon_Jester]

So basically, half of all babies get delivered by Caesarian under that rule, because the due date is based on the average pregnancy duration with roughly half being over and half being under? Wow.

Also, you used the past tense; is there a reason for that?

[Flagg]

Yay, I'm a flaw of nature leading to human suffering! Now I know why life has a 10 yard rusty chainsaw up my ass set on overdrive!

[Flagg]

In my area, Caesarean sections were also on the rise because certain doctors in the area believed their determined Due Date was when a mother would have the child, and if the child wasn't out by the due date, then it was time to cut the baby out.

Certainly sounds like most cunts with a God complex doctors.

[Broomstick]

Researchers estimate cases where the baby cannot fit down the birth canal have increased from 30 in 1,000 in the 1960s to 36 in 1,000 births today.

Er... yeah. OK, how many of those cases are narrow-pelvic women having a child beyond their first? Because in the Bad Old Days they wouldn't have had more than one (a few might have been saved by cesarean).

Historically, these genes would not have been passed from mother to child as both would have died in labour.

Flawed assumption - a narrow pelvis isn't always caused by genes. Two of the other causes off the top of my head are poor nutrition (used to be a killer in the 19th Century) and DES, a medication given in the 1950's and 1960's to prevent miscarriage that lead to birth defects in the children, including a malformed pelvis.

So, without taking the above two factors into account this whole article is meaningless.

Then there are factors that can result in a larger-than-average baby which, again, may or may not be genetic.

[Ziggy Stardust]

Even beyond the case that Lady Tevar describes, in general there have been radical changes in our understanding of, attitudes towards, and practices regarding pregnancy and birth. Caesarean operations are also significantly safer to do now than they were 50 years ago. We also have more detailed ideas regarding when it may be appropriate to do so with respect to the safety of the mother and the baby. It's hard to decouple these types of trends from any theoretical genetic drift.

[Elheru Aran]

Also, you used the past tense; is there a reason for that?

Not Tevar (I'll let her speak for herself) but in my experience, having had one kid and being about to have another, what we've been told by the OB/GYN and various others is that basically now they're rolling back Caesareans to being optional rather than nigh-mandatory. They would rather induce labor now and see how that goes. That said, every practice is different.

[LadyTevar]

Not Tevar (I'll let her speak for herself) but in my experience, having had one kid and being about to have another, what we've been told by the OB/GYN and various others is that basically now they're rolling back Caesareans to being optional rather than nigh-mandatory. They would rather induce labor now and see how that goes. That said, every practice is different.

THIS^^

It was also putting undue stress on the mother, and forcing her to have totally unnecessary surgery when most of the time the child could have been delivered normally without induction. And, as Simon Jestor pointed out, a child's Due Date is mostly guesswork, and often the doctor has the date wrong. It was doctors who wanted to have the baby on THEIR time-table, and not when the child* was ready to come.

*Interesting fact -- it seems that the fetus is the one that triggers the start of labour, not the mother. This may help explain some spontaneous abortions when there was no visible abnormality.

[Elheru Aran]

It's a bit of give and take as far as the timetable goes. With ours, #1 was due on the... I dunno, 4th or something like that. We go to the Dr's that day, they check, not one peep. Dr (actually a RN midwife, but who's counting) shrugged, said to wait and see and if nothing happens, come back in a week and we'd discuss options but she was leaning towards inducing labor after that point as she didn't like to wait too long after the due date (another example of individual variation in practices).

Well we came back in a week, and surprise, my wife was actually in the very early stages of labor that very day. She'd been complaining of back pain the night before... turns out those were probably contractions. So we checked into the hospital and waited. The contractions weren't getting much closer at all, so Midwife suggested we hurry up the process a little bit, we agreed, and they started inducing things. A bit down the way, she got an epidural, and a few hours later, baby popped out. I get to see all that again in about two and a half months. Whee.

[Broomstick]

While the "fact" is that a human pregnancy is 280 days that's really only an average - babies being living creatures they are somewhat variable. Labor is triggered by the baby's development, and it's entirely possible (and completely normal) for some to take 260 days, some 290...

On the other hand, just as delivering a baby too early has risks, delivering too late also has risks. Back when I was still a bun baking in the oven it was impossible to know the exact date of conception, nor were there tests to determine fetal age with any accuracy. Due dates were much more guesswork back then and doctors more inclined to let things go longer - I was three weeks "late", a much longer time that the modern medical world would tolerate these days, but who knows? I may have actually been properly on time.

For a time, the more accurately date of conception and/or age of fetus could be determined the more doctors seemed to be to hold pregnancy to exactly 280 days... never mind that people are variable. I think we're swinging back from that extreme, which is probably all to the good.

[LadyTevar]

I was a full *MONTH* late from the 'due date' of August 30th. My mom finally told the doctor "It's my mother's birthday today and I want this child out!" She was told to drive down to the hospital (another 15miles from the doctor's office) and he'd give them orders to have a room set up for her. When he got there, she was started on the induction, and I was born a little after 9p. I was a fat, healthy pink baby, and had almost none of the white residue that most newborns have on them.

[Flagg]

I was several weeks late, too. But I was a big baby so after my mom went through 48 hours of labor with no me popping out (Right way, wrong way, Army way) they did the C-section.

[ArmorPierce]

Smells like quack science.

1960s to now is one or two generations. Not enough time for noticeable effects on the population. The more likely reason for higher ceasarean's is that doctors are more likely to choose to administer a caesarean compared to back in the 1960s for a variety of reasons, mainly due to better medical procedures, access, and knowledge.

Now, does increased caesareans pose the potential of effecting human evolution, I would say yes. But I am inclined to say that it would not be measurable within 1 or 2 generations.