Low numbers are not the only problem that men encounter when it comes to their sperm. Sperm can also have low motility (meaning that they’re slow), poor morphology (meaning that they’re misshapen), or low vitality (meaning they’re mostly dead). If the semen’s pH levels, viscosity, or liquefaction times are abnormal, it can make conception harder as well. In short, a lot can go wrong.
Females have a similar set of problems pertaining to the production and release of eggs. The female’s reproductive system is far more elaborate than the male’s and is thus more vulnerable to complications. While the majority of these occur in the uterus, thereby impairing a woman’s ability to sustain a pregnancy, some women have difficulty releasing healthy eggs in the first place.
About 25 percent of female fertility problems can be traced to a failure to ovulate a healthy egg. In most cases, we just don’t know what causes the problem, although a few genetic and hormonal syndromes have been identified as culprits. Fortunately, modern science has had reasonable success in coaxing the female reproductive cycle back on track. With carefully timed hormone injections, many women can be induced to ovulate even when their own hormones have failed them. These treatments work so well that more than one egg is released at a time. This has led to a marked increase in the rates of fraternal twins in Europe and North America.
Even when each potential parent produces and releases healthy sperm or egg cells, there is no guarantee they’ll be able to create a pregnancy. First of all, insemination has to be timed very carefully with ovulation or it won’t be successful. In a typical twenty-eight-day menstrual cycle, the fertile period is just three days long in the very best of scenarios, with twenty-four to thirty-six hours being the more common window of opportunity. This means that even perfectly fertile couples usually have to try for months before the female conceives.
The biggest obstacle to getting the timing right is the completely concealed nature of ovulation in humans. Neither the male nor the female knows for sure when it happens. This is in stark contrast to basically all other female mammals, including the other female apes, who conspicuously advertise when they are at the fertile point of their estrous cycle. To be sure, other animals have plenty of sex outside the fertile period, underscoring the many nonprocreative functions of sex, such as strengthening a pair bond. However, when the goal is to have offspring, it sure is convenient to be clear about the best time for conception.
Why is concealed ovulation peculiar to Homo sapiens? There may be adaptive purposes for the concealment; if a man cannot be sure when a woman is ovulating, he cannot be certain that a child is his own unless he sticks with one female constantly. If ovulation were obvious, an alpha male could simply have sex with every ovulating female, spreading his genes widely but not sticking around to invest in the offspring. Thus, concealed ovulation has led humans to form more long-lasting pair bonds and enhanced paternal investment in the offspring. But here, too, a feature of our bodies is also a bug—concealed ovulation adds tremendously to the inefficiency of human reproduction. Other animals know exactly when the fertile period of the estrous cycle is. Humans have to guess.
Most other mammals are so successful at conceiving that females automatically go into the pregnancy cycle right after having sex, even if they’re not actually pregnant. For example, in rabbits and mice, if a vasectomized male copulates with a female, her uterus will prepare to nurture a developing pup for many days afterward, a condition called pseudopregnancy. So successful is sexual reproduction in these animals that any time a female has sex while she’s in heat, her body just assumes conception has occurred.
If human females actually became pregnant every time they had sex during the fertile period, humans would be reproducing like, well, rabbits. But even when the egg and sperm are healthy, and the sperm finds the egg, and fertilization occurs, it is still no sure bet that a viable pregnancy will result. In fact, many of the most error-prone steps lie beyond the moment of conception.
According to the American College of Obstetrics and Gynecology, between 10 and 25 percent of all recognized pregnancies end in spontaneous abortions (miscarriages) within the first trimester (thirteen weeks). That is probably a severe underestimation because it includes only the recognized pregnancies that are lost. From studying fertilization in vitro, we have learned that chromosomal errors and other genetic catastrophes are astoundingly common and these can imperil a potential pregnancy well before it can be recognized. Embryologists estimate that, even with otherwise normal sperm and eggs, 30 to 40 percent of all conception events result in either failure of the embryo to attach to the uterine wall or spontaneous abortion shortly after it has.
Miscarriages beyond the first trimester, while not as common as spontaneous abortions during that period, also plague the human reproductive process. Of pregnancies that make it to the thirteenth week, 3 to 4 percent end before the twenty-week mark. Beyond twenty weeks, miscarriages are usually referred to as stillbirths and occur in less than 1 percent of pregnancies. All told, a staggering one-half of human zygotes—the one-celled union of sperm and egg—don’t make it more than a few days or weeks. Honestly, I sometimes wonder if humans are any more efficient than the mighty oak tree that drops thousands of acorns year after year in the desperate attempt to create one or two saplings.
The most remarkable fact about human fertility is that up to 85 percent of all miscarriages are due to chromosomal abnormalities, meaning that the new embryo has extra, missing, or badly broken chromosomes. Doing the math, this means that when a human sperm and a human egg fuse, the resulting embryo ends up with the proper number of intact chromosomes only around two-thirds of the time. The remaining 15 percent of miscarriages are caused by a variety of congenital conditions, such as spina bifida or hydrocephalus.
Of course, chromosomal problems and other congenital defects become an issue only after a woman has become pregnant in the first place. Sometimes things don’t even get that far. Even when everything goes right—healthy sperm finds healthy egg in the right place and the right time, and the chromosomes commingle correctly to produce a zygote with nothing extra and nothing missing—pregnancy simply doesn’t occur and we have no idea why not. This is called failure to implant, and it happens shockingly often. The developing embryo just sort of bounces off the uterine wall and perishes from lack of nourishment.
And even when an embryo does implant, it sometimes fails to convince the body not to begin menstruation. This means the embryo has failed at its first challenge: preventing the mother’s body from shedding its endometrium (the lining of the uterus and the substrate in which the embryo is living and growing). Embryos have about ten days from the time of implantation to the next scheduled menses, so they typically get right to work secreting a hormone called human chorionic gonadotropin (HCG); this preserves the endometrial lining and thus staves off menstruation, allowing the embryo to continue growing without being thrown out with the proverbial bathwater. Many embryos simply do not secrete enough HCG to prevent the scheduled monthly flow. This means that perfectly healthy, growing embryos are lost in the mother’s monthly menstrual blood for no good reason.
While it is not possible to know for sure, conservative estimates for the percentage of perfectly healthy zygotes that fail to implant or fail to prevent menstruation are around 15 percent. (This is in addition to the one-third of zygotes that do not thrive for reasons we do know.) Some couples experiencing infertility with no known cause may actually be conceiving zygotes just fine; the embryos may simply be failing to take root in the womb.
These glitches in the reproductive system are especially maddening—and painful—for couples trying to conceive. And they are all bug, no feature; there is no justification for the spontaneous abortion of a perfectly healthy embryo or for the apparent failure of seemingly healthy reproductive organs to establish a pregnancy in the first place.
Considering all the challenges facing couples trying to conceive and maintain a healthy pregnancy, it’s kind of amazin
g that anyone makes it through gestation at all. For those who do, one final danger awaits.
Death by Birth
Embryos that are lucky enough to have the correct number of chromosomes, implant successfully, and develop properly through the pregnancy must clear one final reproductive hurdle: childbirth. Thankfully, the advances of modern medicine have significantly alleviated the risks involved in this process, but make no mistake: for most of human history, childbirth was an incredibly dangerous endeavor, and lots of children—to say nothing (for now) of mothers—simply didn’t survive it.
Global statistics are not kept for the percentage of children who die during childbirth itself. Instead, infant mortality rate is generally reported as the percentage of children who do not survive the first year of life—everything between the mother’s labor and the child’s first birthday.
As of 2014, all but one of the major developed countries had an infant mortality rate below 0.5 percent. The one exception is the United States, which, at 0.58 percent, has a higher infant mortality rate than Cuba, Croatia, Macau, and New Caledonia. (This is due in large part to two particular practices by American doctors: the frequent medical induction of labor, which artificially accelerates the natural process of childbirth, and the overuse of cesarean sections. The reason C-sections are performed so often in the United States? Lawyers. Doctors fear being sued on the off chance that a C-section was needed but wasn’t performed. But tragically, these invasive abdominal surgeries often involve many fatal complications of their own.) By contrast, the infant mortality rate in Japan is 0.20 percent, and in Monaco, it’s 0.18 percent.
These are relatively low risks, to be sure, but birth is still one of the riskiest moments of our lives. And in regions where medical practice is far from modern, there is still a high rate of infant mortality—a testament to how far from perfect the human reproductive system is. In Afghanistan, for instance, the United Nations estimates the current infant mortality rate at 11.5 percent. In Mali, it is 10.2 percent.
For readers in the developed world, it is astounding to contemplate that in those two countries, one in ten babies does not survive the first year of life. Three dozen more countries, all of them in Africa or South Asia, have an infant mortality rate above 5 percent.
If we look back in time, even within the wealthiest nations, we see a much higher infant mortality rate than we have now. In the United States in 1955, for instance, more than 3 percent of babies did not make it to their first birthday. That rate is six times higher than it is today. In poor countries, the situation was even worse in 1955 than it is now. There were dozens of countries whose 1955 infant mortality rate was higher than 15 percent, and several whose rates were above 20 percent!
My mother had five children, the first in the mid-1960s. Had she lived in Nepal or Yemen a decade earlier, it is unlikely that all her children would have survived. (This is especially unsettling to me, since I am the fifth.) This is all the more disturbing considering that this high level of mortality occurred within living memory, not way back in the Stone Age. How much worse would things have been in the prehistoric period?
This sad state of affairs is in no way representative of other primates or any other mammals. While chromosomal errors and failures to implant are probably just as high among our ape relatives, miscarriages, stillbirths, and infant deaths during delivery are quite rare in other animals, especially primates. One-year infant mortality for wild animals is difficult to measure with certainty, but the best estimates for the other apes is 1 to 2 percent, making their birthing process several times more dangerous than that of humans in the modern United States but several times less dangerous than that of people in Mali or Afghanistan or the pre-1950 United States. Keep in mind, too, that we are talking about apes in the wild; animals born in their natural habitat usually fare much better than those born in captivity.
In other words, ultrasounds, fetal monitoring, antibiotics, incubators, respirators, and, of course, expert physicians and midwives have all worked together to bring the human infant mortality rate down to what it is for most other species naturally.
Part of why humans are so out of step with other mammals when it comes to childbirth is because human infants are simply born too early. This is due to our species’ massive craniums and the females’ relatively narrow hips. Human gestation time is similar to that of chimps and gorillas, even though humans’ much larger brains require more time and cognitive development in order to reach their full potential. However, the size of the female pelvis limits how large the fetus’s head can grow while still in utero. If it grows too large, there is no way to get it out, and both baby and mother can perish. The compromise is that fetal gestation is cut short, and human babies are born way before they are ready.
The relative sizes of female pelvises and infant heads in (from left to right) chimpanzees, Australopithecus afarensis (of “Lucy” fame), and modern humans. Human infants’ large craniums barely fit through their mothers’ birth canals—one of the main reasons why infant and maternal mortality is high in humans but rare in other apes.
We are basically all born premature. Premature, and completely helpless. The only thing human infants can do for themselves is suckle, and around 5 percent can’t even do that. This, yet again, is not the case for most other mammals (other than marsupials, but they complete their development in a pouch, which is cheating). Baby mammals such as cows, giraffes, and horses hit the ground running—literally. Once they pop out and shake off, they amble around almost immediately. Dolphins and whales are born underwater and, without a moment’s hesitation, swim to the surface to take their first breath with little or no struggle. Humans, however, need more than a year before they can get around on their own, and in the meantime, they are vulnerable to any number of threats.
Human infants are so helpless that it almost seems like there must be a reason for their plight—a reason that, perhaps, could also help to explain why we as a species are so bad at making babies in general. Indeed, our myriad problems with fertility represent such a striking contrast with other mammals’ reproduction that some biologists have wondered if it may actually be an adaptive response to how helpless humans are as infants.
These scientists reason that a reproductive slowdown was required so parents could give children the time and care they needed before the adults reproduced again. In this view, our reproductive problems are not a curse but a blessing. They have the effect of making pregnancies less frequent than they would be otherwise, which in turn means that each child humans do manage to create has a better chance of success because he or she will be the sole focus of parental care for a longer time. In other words, our species’ poor overall reproductive rate may be nature’s way of keeping parental attention on the helpless infant until he or she can stand on his or her own two feet (again, literally).
There is just one problem with this reasoning. If nature wanted humans to space out children, why achieve it through painful and energy-expensive deaths and false starts? Especially when there is a far easier way: the female body could just delay the postpartum return to fertility for a longer period. That’s what many species, including our close relatives, do. In gorillas, average birth spacing approaches four years, except when a nursing infant dies, at which point the mother almost immediately goes into estrus. In chimps, the average spacing is more than five years, and in some orangutans, average birth spacing is nearly eight years! Continued childcare, primarily nursing, inhibits the ovulation-menstruation cycle in these apes, leading to sensible spacing intervals. Mothers and fathers are free to provide the care to their infants and juveniles for as long as the offspring need it.
Not so with our species. Humans keep pushing them out and hoping for the best. Because all our closest relatives have longer postpartum delays in fertility, it is likely that our common ancestors did also. We are the outliers, in other words. This means that, through the evolutionary history of humans, we have seen a decrease in fertility spacing in females. T
his doesn’t alleviate the problem of helpless infancy; it compounds it by throwing additional babies into the arms of parents still struggling to wean previous ones.
The leading explanation for human females’ quick return to fertility is that, as human tribes grew bigger, they shifted toward communal parenting. When children were raised cooperatively in large extended families, the parenting burden was shared, and females didn’t have to delay the next pregnancy. Furthermore, hunting and gathering became more efficient due to the increasing intelligence, communication, and cooperation of our human ancestors, and this allowed some women to concentrate solely on child-rearing. Not surprisingly, most of the proponents of this theory are men.
Sexist implications alone are not enough for researchers to discard a scientific hypothesis—but there are other reasons to reject this one. For instance, I find this explanation to be insufficient because, at best, it explains only the decrease in birth interval. Humans have issues throughout the entire process of reproduction. If, over the past million years, human advances allowed for an uptick in fertility, why would birth timing be the only sign of that increase, while all other aspects of fertility continued to worsen?
Human Errors Page 11