Why the Long, Difficult Birth for Humans?

Labor for first time moms averages somewhere between 10 – 20 hours. Among other Great Apes, such as Chimpanzees, labor takes on average two hours. In addition to the time in labor, human labor is described as extremely difficult compared to other primates.

What characteristics contribute to such disparate childbirths in humans and other primates?

1. The Obstetric Dilemma

The most significant tradeoffs in human evolution are evidenced at the time of birth: efficient bipedal locomotion promotes a relatively narrow pelvis, while delivering relatively large-brained babies selects for a broad pelvis. In other words, the narrow pelvic opening that is good for efficient upright walking is not so good for the delivery of large-brained humans.

The interaction of these features in our evolutionary history has shaped a female pelvis that reflects a compromise in these selective pressures: the constraints on the human pelvis due to walking on two legs, combined with the exceptionally large brains of human babies make human birth especially long and painful.

2. Bipedalism Promotes a Narrow Pelvis

Humans are the only living mammal to walk on two legs, and bipedalism requires narrow, short, squat, and basin shaped pelvic anatomies. In contrast, quadrupedal animals (like our closest relatives, chimpanzees) have pelvises that are long and two dimensional in appearance. Compare image A (the pelvis of a chimpanzee) to image C (the pelvis of a human female).

Comparison of pelves from (A) Pan, (B) Australopithecus, (C) Human female, and (D) Human male (from Lovejoy, 2005, figure 5).

Comparison of pelves from (A) Pan, (B) Australopithecus, (C) Human female, and (D) Human male (from Lovejoy, 2005, figure 5).

Notice how long the chimpanzee (image A) pelvis is compared to the squat pelvis of a human female (image C)? That humans walk on two legs is one explanation for the pronounced differences between humans and chimpanzees.

3. Big Brains and Birth

One important characteristic of primates as a group is a high encephalization quotient, or a large head and brain relative to body size. For most primates, this means that neonates at birth have heads that are close to the size of the maternal birth canal through which they must pass, making birth far from easy.

From Rosenberg and Trevathan (2002); based on Shultz (1949).

From Rosenberg and Trevathan (2002); based on Shultz (1949).

The image above describes the relationship of the maternal pelvic inlet (outer white ovals) and size of the baby’s head (dark solid ovals).

Think of a hard boiled egg. For gorillas and chimpanzees, the baby’s head is the size of the yolk, while the mother’s pelvic opening is the size of the egg white. Though baby must pass through the muscles and tissue of the birth canal, there is no bony resistance from the mother’s pelvis. For humans, imagine the hard boiled egg as the shape of the pelvic opening, and a golf ball as the baby’s head.

4. Passage through the Birth Canal

It’s not just the tight squeeze that makes birth difficult – the human birth canal isn’t a constant shape in cross-section. Thanks to bipedalism, the the birth canal is twisted in the middle, while the birth canal is described as a “relatively straight tube” for quadrupedal primates. A monkey baby has a tight squeeze through his mother’s pelvis, however her birth is significantly less difficult than a human baby’s. This is because, unlike the complicated cross-sectional area of the human birth canal, a monkey’s birth canal maintains the same cross-sectional dimensions from front to back.

Image by Karen R. Rosenberg and Wenda R. Trevathan

Image by Karen R. Rosenberg and Wenda R. Trevathan

As a human baby makes her way down the birth canal, she enters the birth canal at it’s widest side-to-side, relative to the mother’s body. But midway through, the orientation of the birth canal shifts 90 degrees, and the baby must now navigate a series of twists and turns so that her largest dimensions – the head and shoulders – are aligned with the largest areas of the birth canal.

The baby must rotate through the changing dimensions of the birth canal twice in order to navigate and pass through the differing pelvic dimensions through the birth canal’s entrance and exit. Our unique mechanism of birth is one of the distinguishing features of human birth, and adds an additional source of difficulty for so many mothers and babies.

5. Differing Degrees of Cervical Dilation

Among primates, human babies have exceptionally big brains. Because of this, the female cervix must dilate three times as much as other primates in order to deliver the big-brained, broad shouldered baby.

Image via Dr. Cheryl Knott

Image from Dr. Cheryl Knott via

Chimpanzee mothers must dilate around 3.3 centimeters before they’re able to give birth. Human mothers must dilate 10 centimeters before they’re able to deliver. The extra 6.6 centimeters of dilation takes significantly more time, but later stages of dilation are also experienced as significantly more painful. As described in the Mayo Clinic, the early stages of labor (0 – approximately 4cmaren’t always experienced as particularly uncomfortable, and certainly don’t require the same degree of attention as later stages of labor. Later stages of dilation (around 8, 9, 10 centimeters) leading up to transition and delivery are when women experience the most pain.

In Sum: the Obstetric Dilemma

Researchers eloquently describe the interaction of demands as a “tug of war” exerted on the maternal pelvis, which makes human birth significantly more risky than birth in other primates and mammals. Others have described this tradeoff as a scar of human evolutionary history, or the obstetric dilemma.

The Good News: Delivering with Company

Unlike most primates who seek solitude during parturition, virtually all women cross-culturally seek out assistance during birth from relatives, doulas, obstetricians, midwives, and other supports. While there are stories of women who had successful solitary births, studies of childbirth across cultures find that these stories are exceptional. Across cultures, women seek out both physical and emotional assistance in labor and delivery.

If you’re pregnant, consider giving birth with continuous emotional support: from a partner, family member, friend, or doula. A woman’s desire to be surrounded by familiar “others” has deep roots in our human lineage, is associated with fewer birthing complications.

Please feel free to contact me at mikaelawapman@gmail.com, or in the form below:




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Aiello, L., Wheeler, P., (1995). The expensive-tissue hypothesis: the brain and the digestive system in human and primate evolution. Current. Anthropology. 36, 199–221.

DeSilva, J.M., Lesnik, J.J. (2008) Brain size at birth throughout human evolution: A new method for estimating neonatal brain size in hominins. Journal of Human Evolution 55:1064–1074.

DeSilva, J. M. (2011). A shift toward birthing relatively large infants early in human evolution. Proceedings of the National Academy of Sciences of the United States of America108(3), 1022-7.

Ellison, Peter T. On Fertile Ground. Cambridge, MA: Harvard University Press, 2001. Print.

Hrdy, S.B. (2009). Mothers and Others. Harvard University Press: Cambridge Press.

Hrdy, S. B. (2005). Comes the child before man: how cooperative breeding and prolonged postweaning dependence shaped human potential. in Hunter-Gatherer Childhoods: Evolutionary, Developmental & Cultural Perspectives. Edited by: B. S. Hewlett and M. E. Lamb. Aldine Transaction: New Brunswick, 65-91.

Hodnett  ED,  Gates  S,  Hofmeyr  GJ,  Sakala  C,  Weston  J  .  “Continuous support for women during childbirth.”  Cochrane Database Syst Rev. 2011 Feb 16; (2):CD003766.

Hogan MC, Foreman KJ, Naghavi M, Ahn SY, Wang M, Makela SM, Lopez AD, Lozano R, Murray CJ. 2010. Maternal mortal- ity for 181 countries, 1980-2008: a systematic analysis of pro- gress towards Millennium Development Goal 5. Lancet 375:1609–1623.

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Rosenberg, K. & Trevathan, W. (2002). Birth, obstetrics and human evolution. BJOG: an International Journal of Obstetrics and Gynaecology, 109, 1199-1206.

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First Time Moms: Consider Perineal Massage

Tearing during childbirth is common (more than 70% of women experience perineal trauma in childbirth) especially for first time vaginal births, and in vacuum or forceps assisted deliveries (Beckman and Stock, 2014Aashiem, Nilsen et al. 2011). Studies have found that perineal massage before birth has a positive effect on the incidence of perineal trauma at birth among first time moms. Second time moms experience a significant reduction in pain three months postpartum.


Pregnant [39 weeks]” by Kevin Dean is licensed under CC BY 2.0

What is perineal massage? 

Perineal massage stretches the inner tissue of the lower vagina. Perineal massage teaches you to respond to pressure in your vagina by relaxing your pelvic floor, and can increase muscle and tissue elasticity. It is also thought to enhance the hormonal changes that soften the connective tissue in late pregnancy. I include instructions on how to massage your perineum later in this post.

How do I go about it?

Typically, women start massaging around 34-35 weeks of pregnancy, and are encouraged to spend 5 – 10 minutes 1-2 times a week. Recent studies indicate that the more frequently women use perineal massage, the less likely they are to experience the benefits. Specifically, women who did perineal massage 1.5 times a week experienced a 17% reduction in perineal trauma and a 17% reduction in episiotomy. In contrast, women who massaged between 1.5-3.4 times a week experienced only an 8% reduced risk of perineal trauma. So, the less frequent the massage, the more beneficial the outcomes. 

The research:

In a Cochrane Review, women who massaged their perineum once or twice a week beginning at thirty-five weeks of pregnancy were compared to women who did nothing. Researchers found that women who were assigned to do perineal massage experienced a 10% decrease in risk of tears requiring stitches (aka perineal trauma), and a 16% decrease in the risk of episiotomy, however these findings were only significant for first time moms.

Although second time mothers did not experience the same reduction in risk of stitches and episiotomy in childbirth, second time mothers reported a significant reduction (32%) in the incidence of pain at three months postpartum. More frequent massage in second time moms was associated with a reduction in perineal pain postpartum.

So, for first time moms only, perineal massage 1.5 times a week during pregnancy has been found to decrease the risk of episiotomy, as well as a decrease the risk of trauma requiring stitches. 

For second time moms, frequent perineal massage (between 1.5-3.4 times a week) is associated with a reduction in pain postpartum.

How to do a perineal massage:

  • Find a comfortable position (legs bent outward while lying on the bed, standing with one foot on the edge of a bathtub or counter, sitting on the toilet)
  • Use unscented oil such as olive, sunflower, or sweet almond oil. Insert one or both thumbs in the entrance of the vagina.
  • Pressing towards the back of your vaginal wall, move your thumbs back and forth in a U-shape, focusing on relaxing your muscles at the same time. First downward for 2 minutes, and then sideways for 2 minutes.
  • Massage for around 5 minutes each time. Remember your breath as you do this – count to four on your inhales, hold, and count to four on your exhale.
  • You can try pushing harder each time you do a perineal massage.

Or if you prefer, a medical explanation how-to: “Digitally stretch perineal tissues by inserting lubricated fingers 1.5 inches into the lower portion of the vagina and slowly massaging downward in a U-shaped movement” (Beckmann and Stock, 2014).

Questions? Feel free to check out any of these sources: 

Where are the safest places in the world to give birth?

Tracy Cassels from Evolutionary Parenting recently posted on new research on the current status of maternal mortality around the world.

Where would you rather give birth – in the USA or Albania?  Most of you would probably say the USA yet your chances of survival would then be lower.  Yes, you as a mother are more likely to die from childbirth-related problems in the USA than Albania.  

Do you find this surprising? Here is the research:

New research looking at maternal mortality is out* and the findings are not good for those in the American system.  In 1990, the USA was ranked 22 in maternal health and mortality.  This year, the USA ranks 60th after dropping from 50th in the last assessment.  In fact, the US is one of only 8 countries to have seen a rise in maternal morality, up from 17.6 in 2003 to 18.5 deaths/100,000 in 2013 (the other 7 were Afghanistan, Belize, El Salvador, Guinea-Bissau, Greece, Seychelles, and the South Sudan).  In 1987 that number was 7.2/100,00 […].

Canada and New Zealand have maternal mortality rates that are half of the USA.  The UK has one that is a third.  Australia’s is a quarter of the USA’s.  Albania, Bosnia and Herzegovina, Bulgaria, Croatia, Romania, Kuwait, Lebanon, Iran, and Serbia all have rates that are lower than that of the USA.

What can explain such high rates of maternal mortality? No, it’s not the rise in homebirth.

One of the main factors is a rise in certain diseases that make pregnancy riskier, like diabetes and hypertension.  There are also other heart and neurological diseases that are on the increase that necessitate higher-risk pregnancies.

This possible explanation leads Tracy to ask:

If most of these increases are due to increases in diseases, why are other similar, developed countries not showing the same problems?  How is all the medical technology in the world failing American mothers?  At this stage, it’s hard to know what parts actually work, but clearly the system taken together doesn’t.  Perhaps it’s the lack of integration, the lack of affordable care, or the focus on treatment instead of prevention.  Likely it’s a mix of all of it and more.


Oh – to answer the question of where you would be safest giving birth: Iceland with a maternal mortality rate of 2.4/100,000 (nearly 1/8 that of the USA).


Read the whole post here.



Image via Every Mother Counts

* Kassebaum NJ et al. Global, regional, and national levels and causes of maternal mortality during 1990-2013: a systematic analysis for the Global Burden of Disease Study 2013.  The Lancet 2014; doi:10.1016/S0140-6736(14)60696-6.

Riddle me this

A father and his son are out driving and are involved in a terrible accident. The father is killed instantly, and the son is in critical condition. The son is rushed to the hospital and prepared for an operation that could save his life. The surgeon comes in, sees the patient, and exclaims, “I can’t operate, that boy is my son!”

Who is the surgeon?

Take a moment to consider all possible solutions to this question. Really, take some time before reading on.

Image is from the Marion Fayolle of the NYT

Image is from the Marion Fayolle of the NYT


Maybe the surgeon is the boy’s gay second father! Maybe it is a case of adoption! Great guesses.

But did you consider that the surgeon could be the boy’s mother? If not, you’re in the overwhelming majority.

This is a famous proverbial riddle. The riddle has been around for decades, yet it continues to stump those who have never been exposed to the riddle. My undergrad senior psychology thesis explored how individuals respond to this classic riddle.

Deborah Belle, Professor of Psychology at Boston University, and I presented this riddle to two groups of participants: 197 BU Psychology students, and 103 children between the ages of 7 – 17 years old. In addition, we presented participants with a questionnaire in which we asked about their personal experience with women physicians and other experiential, attitudinal, and demographic variables.

In both groups, only a small minority of participants were able to imagine that the surgeon could be the boy’s mother: 14 percent of BU students, and 15 percent of children.

Screen Shot 2014-06-28 at 10.20.36 AM

Curiously, life experience (such as exposure to female doctors, working mothers, etc.) had no association with how one responded to the riddle. Even the majority of participants who have working mothers, or even mothers who are doctors were unable to imagine a surgeon mother in their solutions to the riddle. Other irrelevant factors include: gender of participant, personal exposure to female physicians, having had an employed mother, political views (conservative-liberal), social class growing up, and modern sexism scale scores.

Participants may have been stumped by the”surgeon mother” answer to the riddle, however, they had many, many ingenious explanations to the classic riddle. Some of these include:

  • Nurse has Alzheimer’s and is convinced the patient is her daughter.
  • Maybe the father who drives with the boy is a priest. They are often referred to as father.
  • The nurse is hallucinating, brainwashed, or delusional.
  • It was a dream.
  • What? No. Either the surgeon is lying, the paragraph is lying, or both.
  • The dad laid down and officials thought he was dead but he was alive.
  • It was a robot
  • I think the mom got back to life because maybe she could have just stopped breathing and then a miracle happened

More frequent responses to the riddle include: gay parent, adoption, mistaken identity, or step parentIn fact, participants were almost three times as likely to suggest gay parents as an explanation to the riddle as they were to come up with a female surgeon (37% versus 14%, respectively).

What makes it so difficult for participants to imagine a surgeon mother as a solution to the riddle? 

Gender schemas may explain our findings. Gender schemas are generalized, nonconscious hypotheses about the complex world around us, that don’t necessarily reflect one’s personal values or life experience. Gender schemas are very powerful, and resistant to change despite contradictory information. So, people are more likely to notice the things that fit into their schema, and re-interpret or distort those that contradict the preexisting schema. For a more flushed-out explanation of schemas, check out this interview with Virginia Vallian and the NYT, here.

Using gender schemas to interpret the riddle study’s findings, it makes sense that there is no statistically discernible association between participants who grew up with a surgeon mother, or had an operation with a surgeon mother, and the way these participants respond to the riddle. Men and women hold the same gender schemas, and begin acquiring them early in their childhood – explaining why there was no noticeable difference between how males and females responded to the riddle.

Gender schemas, one aspect of the schemas we each acquire, are generalized hypotheses about men and women. These hypotheses affect our expectations of men and women, our evaluations of their work, and their performance as professionals. As evidenced by the growing body of research, gender schemas tend to focus on a woman’s reproductive functioning, associating professional competence with men. Although experience does contribute to one’s schemas, it is far less than you’d expect – exactly what we our data suggested in our riddle study.

So what? How does this silly riddle interact with our lives?

Gender schemas eek their way into our lives in subtle, profound ways.


For example, you’re reading the newspaper:

A Berkeley physicist reported today that…


(scroll down)




                                                                                                 her research team had discovered…


Other studies point to the pervasiveness of gender schemas. Moss-Racusin et al. (2012) found significant gender bias among the Science Department Faculty, as Yale faculty participants rated “male” applicants as significantly more hirable, competent, more deserving of mentoring and a higher salary than identical “female” applicants, while the “female” applicants were rated as more likable. Faculty participant bias was independent of age, gender, scientific discipline, and tenure status.

These studies contribute to the large body of research highlighting the problems facing women in the workplace. One fundamental aspect of schemas is their resistance to change or adaptation, despite contradicting information. In the case of gender schemas, men are perceived as more competent and hirable, while women are perceived as warm and nice.

How then, can we confront the effects of these schemas?

Eternal vigilance. Self-awareness that we each hold certain biases about men and women, as we approach job applicants, writers, authors, scientists, politicians.. every individual we encounter. Our perceptions and judgements of individuals are colored by gender schemas, and our judgements are prone to error.

Gender schemas do change, though at a slow, unhurried pace.


Read other coverage of our research here, or here