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Evolution has handled even bad births very well in some sneaky ways – ever hear of ‘birth amnesia’ (“you won’t remember the pain”)? How about mothers saying ‘I’d do it all again, it was worth it’ – brain chemicals evolve, too. Love them endorphins, and oh, how about oxytocin and estradiol, both of which also contribute to the ‘scr*w the pain, I’d do it again for a baby’ response.
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“Pure” evolution is confounded by medicine, technology, society, and culture. 11 lb babies used to die or kill their mothers or both. Now we c-section. Women can have multiple babies (regardless of difficult births) by choice, not because it is easy, but because they survived, and want children for other reasons. Other women have easy smooth short labors but may only have one child, not because it was hard, but for other reasons. A really bad labor experience can be moderated by medicine, making a difficult birth no longer a deciding factor in decision-making regarding reproduction, nor in results of the labor (dead baby or mother). Even if evolution happened that fast, these would really mess with the progress of it.
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Birth is already highly evolved in humans. There’s not much room for error at this stage – if head size increases much more, pelvic size changes to accommodate would make it hard to walk effectively. It is currently a fair balance between wide hip-joint placement (allowing for larger pelvic opening) and mechanics of birth. For instance, the clever spiral the baby does on the way out – starting sideways to fit the head into the pelvic opening, turning to get the shoulders through, turning again to get head and shoulders through the birth canal with the least damage. And also the loosening of the pelvic joints (the one in front, and the two in back) to allow the pelvis to flex and open to increase the size of the passage. (trust me, those joints can really MOVE once they loosen – I’m a limping testiment to that right now, at 6 days from baby 2’s due date!) Not to mention baby head joints not being solid (allowing for cranial molding/cone-head, making the baby smaller temporarily), and babies being born extremely early in development compared to other species. Sounds pretty damn evolved, to me.
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Labor and birth may not be biologically set up to be agonizing. Animal models indicate that labor runs from slightly uncomfortable to relatively painful under normal conditions, but not the screaming agony so often depicted in media presentations. And animals haven’t evolved past that despiteplenty of time to do so – there doesn’t seem to be much reason to go beyond that level (you don’t want it so easy that they babies fall out before they are done cooking, right? And as has been stated, freedom from pain is not the same as survival.).
There are theories abounding about why labor is so painful in modern times, many having to do with ‘we made it that way’ – NOT biological, but cultural/social, etc. In fairly ancient times (Greek antiquity, say), normal labor wasn’t seen as such a terror, though abnormal labor certainly was, and there was also the risk of death, nothing to sneeze at, as far as anxiety is concerned. Soranus and Hypocrates recommended only minor pain meds, such as willow (aspirin) and hemp (y’all know what that is) for normal labor, (plus non-medicinal support such as bags of warm oil used as both compresses and body support)… and even Pliny the Elder (who dealt with a lot more of the folk-medicine approach) discusses wide varieties of relief methods for extended or abnormal labors, and barely mentions normal ones (at least not that I can find). Soranus very explicitly states that emotional support, reassurance, and encouragement ease labor immensely, and specifically instructs midwives to sit before the woman on the birth chair and tell her the birth will be easy, in order to ease the process. (Soranus: Gyn. 2.3.5, 4.2.2., according to the most scholarly paper on the topic I can find) The worst labors according to Soranus being those of women who didn’t believe they were pregnant in the first place, or those who were anxious, fearful, angry, or emotionally upset. Severe pain, therefore, may be largely a result of expectations and personal reactions at the time (though I woudn’t say always, certainly!). Stress makes pain worse, expecting pain makes one stressed, fear of pain makes one stressed, etc. The whole Dr. Grantley Dick-Read natural childbirth thing (fear-tension-pain cycle). Just by being self-aware creatures, we can make labor worse. Animals have it easier because they don’t ‘know better’, not because they necessarily handle pain better in some biological way.
Personally, I never expected birth to be painful. It wasn’t, as long as I kept out of the way and let my body do its thing. Not comfortable, either, but far from being an agony (gas pains from eating nothing but ice chips and popsicles were worse, trust me!). Labor was welcome, expected, and anticipated to be effort, not pain. By comparison, a miscarriage at 8 weeks, which I fought both physically and emotionally, hurt like bloody freaking hell. If I were an animal, I wouldn’t have been fighting it, and it probably wouldn’t have hurt so bad. Experiences vary widely (as noted by Dangerosa), but reasonable/tolerable labors seem to me to be fairly common even among women who expect worse, and far from enough to cause an evolutionary swing of any sort (by, say women killing themselves rather than do it again).
Using the animal model, birth isn’t excruciating - though if you’ve ever watched a dog or cat birth, and you know anything about how animals cope with discomfort, you’d say it isn’t a pleasant happy pleasurable experience, even if not agonizing. Dogs pant and sometimes yelp with the pup’s expulsion. Cats purr (a sign of stress, not happiness). “Ouch,” not “SHRIEK!” The animal model is that labor is hard work, uncomfortable, and may be painful, but isn’t an agony. Agony is a result of fear/distress/anxiety or of abnormal labor process, or of a specific set of personal conditions including pelvic proportion, positioning of the fetus, and pain threshold. Most of that is not terribly relevant to evolution, except as a side factor of our intellect and self-awareness – we’re able to reason enough to get in the way of the process, or to repeat a negative process because of a positive outcome (baby).
Under animal model conditions, evolutionarily speaking, birth experience is hardly a hindrance to the species. Add a technology/skill like hypnosis/relaxation, or pain meds, and you’ve got quite a good shot at a vastly more pleasant experience than you’ll see on TV, and hardly something to impact us as a species.
IMHO, evolution in humans is way too complex to even ask the questions in the OP, even if pain was a determining factor in evolution (which is doesn’t seem to be). And besides, I don’t think the assumptions behind the question are that valid, either (birth as agony).
However, I have also heard the related idea that menstrual pain is a sham, and I can tell you that it is the worst pain I have ever experienced, worse than slamming my hand in the car door, worse than breaking my wrist. Some of my friends have “minor discomfort” and that’s all…and my horrific cramps did ease quite a bit as I got older. (Meaning over 25.) I have also heard that those who have horrible menstrual cramps have less trouble with labor, as they know what it feels like and can handle it. I wouldn’t know.