Large headed women found to have pelvis shape to allow for delivery of large headed babies

April 21, 2015 by Bob Yirka report
The skull of a newborn must fit through the female pelvis at birth -- it has to be as big as the diameter of the birth canal. Credit: Copyright: Magdalena Fischer

(MedicalXpress)—A pair of researchers, one with the University of Oslo, in Norway, the other with the University of Vienna in Austria, has found that women with large heads tend to have birth canals that are shaped differently on average to accommodate what will likely be babies being born with larger heads. In their paper published in Proceedings of the National Academy of Sciences, Barbara Fischer and Philipp Mitteroecker describe how they analyzed multiple human skeletons and noticed a trend—women with large heads had slightly differently shaped birth canals.

Big heads do not always mean large egos—some people actually have heads that are physically larger than average, which causes some other people, mostly women, to wonder about how things went when that baby was being delivered—a large , it would seem, would have more difficulty passing through the pelvic bones that make up the birth canal, causing the mother more distress than for mothers delivering smaller headed babies. But nature has found a way to help, it appears, as the two researchers discovered.

As part of a larger study to try to understand why it is that female human beings have on average narrow hips and thus a small , relative to baby size, the researchers examined 99 , looking for clues. While they were not able to answer the question of why the female pelvis has not grown larger to accommodate the larger head that has evolved to hold our growing brains, they did spot a trend—they noticed that women with larger heads tended to have a that was structured slightly differently that would help in allowing passage of a baby with a larger than average head. A nice adaptation, they note, for such women, but it does not explain why the female pelvis has not evolved along with the brain—thanks to modern medicine, many more women today survive childbirth than at any other time in history, but if the pelvis had widened over the past generations, than women giving birth would not be at so much risk in the first place.

With the use of 3-D data, the authors identified a relationship between the shape of the pelvis, the body size and head circumference. Credit: Copyright: PNAS

The researchers note that the pelvis actually serves two purposes for , reproduction and locomotion—they suggest that nature has preserved the smaller pelvis because of some locomotive advantage, though no one has been able to show what that advantage might be.

Explore further: Research refutes long-held theory: Mother's metabolism, not birth canal size, limits gestation

More information: Covariation between human pelvis shape, stature, and head size alleviates the obstetric dilemma, Barbara Fischer, PNAS, DOI: 10.1073/pnas.1420325112

Abstract
Compared with other primates, childbirth is remarkably difficult in humans because the head of a human neonate is large relative to the birth-relevant dimensions of the maternal pelvis. It seems puzzling that females have not evolved wider pelvises despite the high maternal mortality and morbidity risk connected to childbirth. Despite this seeming lack of change in average pelvic morphology, we show that humans have evolved a complex link between pelvis shape, stature, and head circumference that was not recognized before. The identified covariance patterns contribute to ameliorate the "obstetric dilemma." Females with a large head, who are likely to give birth to neonates with a large head, possess birth canals that are shaped to better accommodate large-headed neonates. Short females with an increased risk of cephalopelvic mismatch possess a rounder inlet, which is beneficial for obstetrics. We suggest that these covariances have evolved by the strong correlational selection resulting from childbirth. Although males are not subject to obstetric selection, they also show part of these association patterns, indicating a genetic–developmental origin of integration.

Related Stories

Research refutes long-held theory: Mother's metabolism, not birth canal size, limits gestation

August 27, 2012
New research by a University of Rhode Island professor suggests that the length of human pregnancy is limited primarily by a mother's metabolism, not the size of the birth canal. The research, published in the Proceedings ...

Women born early at greater risk of delivering preemies, study suggests

April 10, 2015
(HealthDay)—Women who were born prematurely may be more likely to deliver their own babies early, a new study suggests.

Virtual childbirth simulator improves safety of high-risk deliveries

November 29, 2011
Newly developed computer software combined with magnetic resonance imaging (MRI) of a fetus may help physicians better assess a woman's potential for a difficult childbirth. Results of a study using the new software were ...

Pelvic widening continues throughout a person's lifetime, study

May 25, 2011
By the age of 20, most people have reached skeletal maturity and do not grow any taller. Until recently it was assumed that skeletal enlargement elsewhere in the body also stopped by age 20.

Recommended for you

Sleeping position linked to the risk of stillbirth

November 20, 2017
Pregnant women who go to sleep on their back during the later stages of pregnancy face an increased likelihood of suffering a stillbirth, according to new research.

Study in mice finds dietary levels of genistein may adversely affect female fertility

November 14, 2017
Exposure to the phytoestrogen genistein prior to conception may adversely affect female fertility and pregnancy outcomes, depending on the dosage and duration of exposure, a new study in mice suggests.

IUDs may have a surprising benefit: Protection against cervical cancer

November 7, 2017
Considered a safe and highly effective contraception method, intrauterine devices (IUDs) may also be quietly offering protection against the third-most common cancer in women worldwide. A new study from the Keck School of ...

Increasing rates of chronic conditions putting more moms, babies at risk

November 7, 2017
Pregnant women today are more likely to have chronic conditions that could cause life-threatening complications than at any other time in the past decade - particularly poor women and those living in rural communities, a ...

First time mums with an epidural who lie down more likely to have a normal birth

October 18, 2017
Adopting a lying down position rather than being upright in the later stages of labour for first-time mothers who have had a low dose epidural leads to a higher chance of them delivering their baby without any medical intervention, ...

Mice delivered by C-section gain more weight than those delivered naturally

October 11, 2017
Mice born by Caesarian section gained on average 33 percent more weight in the 15 weeks after weaning than mice born vaginally, with females gaining 70 percent more weight.

2 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

SciTechdude
5 / 5 (1) Apr 21, 2015
All our child care support and keeping people alive who would otherwise have died in the process (All good as far as anyone is concerned) are probably stunting general evolution of traits that would help both mother and child survive childbirth, because we have behaviorally evolved to cope with it instead. Otherwise all of those people who were likely to die in child birth would have, and their kids would have never bred, etc.
Gigel
not rated yet Apr 22, 2015
With intelligent beings technology may be more successful than random evolution, which is why it probably appeared.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.