It's no secret that childbirth is a very painful, sometimes long process to endure. When we look at our animal counterparts, however, it seems they can give birth relatively easily and don't exhibit as much stress as we do. Researchers questioned why it is that we humans suffer so much more and often need medical intervention, while other species are basically unscathed. It turns out it comes down to one thing: evolution.
You see, millions of years ago, when human-like ancestors and relatives needed to procreate, their bodies were much more equipped for the task. Hip bones were wider and the process is thought to have been generally quicker. Even looking at species of today, such as chimps, childbirth doesn't seem to be as big of a deal, nor does it need as long of a recovery time as it does for us.
One explanation for this puzzle is that our ancestors walked on all fours, whereas today, we're bipedal- meaning we walk on two legs. In order for this to occur, our pelvis and hip bones needed to adapt accordingly to support the weight of our torsos. Consequently, our hips became denser, narrowing access to the birth canal.
In addition, as we have evolved, we've become much more complex as a species. With this complexity came intelligence and the need for a larger store-house (a.k.a. brain). Our skulls have become larger to support a larger brain. Evolutionary scientists have found evidence that ancestral babies were able to come straight out of the birth canal, rather than making the numerous twists and turns that our babies need to accomplish to exit today.
With all of that evidence being present, however, it is not a hard fact that childbirth is always harder today than it was in the past. In fact, looking at some women today and some skeletons of women in the distant past, in some cases it was actually harder back then. Evolutionarily speaking, though, as a general population on all ends of the timescale, childbirth has progressively been getting harder- give or take some exceptions.
Both today and in the past, some women are just generally better built for childbirth which explains why some women can deliver babies with little to no pain in no time at all, whereas other women spend days in agonizing pain. What remains a mystery is what childbirth is going to look like many years from now as we continue to progress as a species.