Implications for Pregnancy
A number of adaptations have evolved in women to deal with a tremendous set of physiological and psychological pressures during the 9-month gestational period and labor.
Pregnancy lasts approximately 9 months and places many demands on women’s bodies. First, growing a fetus is tremendously costly in terms of energy needs, and it also places physical strains on a woman’s body. Second, pregnancy greatly inhibits locomotion and makes women vulnerable to threats. This entry will address (a) the basics of pregnancy, (b) two hypotheses about the length of gestation, and (c) the psychological implications of pregnancy.
The Basics of Pregnancy
Gestation begins after copulation during a woman’s fertile window when sperm fertilizes the egg. The zygote (fertilized egg) travels down one of the fallopian tubes as it undergoes rapid cell division before implanting into the uterine wall. For the next 10 weeks, during the first trimester of pregnancy, the embryo and all of the necessary structures for fetal development (placenta and umbilical cord) then develop. After this point, the embryo is typically referred to as a fetus.
Women typically carry only one fetus during a pregnancy, because usually only one egg is released from one of the ovaries during ovulation. However, sometimes women will carry multiples. Twins are most common and can either be dizygotic or monozygotic. Dizygotic (fraternal) twins result from two eggs being released and fertilized during ovulation. Monozygotic (identical) twins result from a single fertilized egg that splits early on in the pregnancy.
Women undergo several physiological changes to accommodate the pregnancy. Endocrinologically, women experience increases in both progesterone and estrogen throughout gestation. Further, women’s immune systems become suppressed so that the genetically dissimilar embryo can develop without being rejected by the mother’s body (Navarrete et al. 2007). Women typically gain weight during pregnancy as the fetus develops, the uterus enlarges, the placenta becomes filled with amniotic fluid, and the body retains extra fat and water. Although women typically do not start lactating until a couple of days after giving birth, women’s breasts develop throughout pregnancy to be able to produce milk, and the areolae typically become larger and darker. Women’s blood volume increases by up to 50%, and they experience a number of other cardiovascular and hematological changes.
Consequently, pregnancy brings several challenges. Women need to increase their protein and calorie intake in order to support the development of the fetus as well as all of the other necessary structures (e.g., placenta and other uterine structures, breasts). At the same time, pregnancy makes locomotion difficult, and women become more vulnerable due to the increased necessary effort (or potential inability) to acquire resources and avoid harm. Further, if things do not go well during the pregnancy, women may experience medical problems or can miscarry. Both of these outcomes are very costly.
Not only is the 9 months of gestation physically costly to women, but giving birth is also an ordeal. A birth is considered full-term if the gestational age of the fetus is 37–42 weeks. Modern humans and other hominins (extinct human species and all immediate ancestors including members of the genera Homo, Australopithecus, Paranthropus, and Ardipithecus) have birth canals that are different from other animals in that they are wider sideway (hip to hip, transversely) than front to back (anteroposteriorly). These physiological constraints mean that babies have to rotate as they are moving down the birth canal; first they face sideway to align their head with the birth canal and then they turn parallel to the mother for the rest of their body to move through the birth canal (Rosenberg and Trevathan 2002). The turning process necessary to align the infant’s body properly with the mother’s birth canal makes the birthing process more complicated.
Further, childbirth is usually painful and can be dangerous as babies’ heads and shoulders are large in proportion to women’s rather narrow birth canals. Unlike other primates for whom labor lasts about 2 h, first-time mothers typically are in labor for 9 h (Albers 1999), and about 1% of babies end up having shoulders too large to pass through their mother’s birth canal (Rouse et al., 1996). Although this would have been deadly in ancestral times, this problem can be solved with a cesarian section and other modern medicinal interventions. Despite the physiological costs and risks associated with gestation and parturition (labor and birth), selection pressures on women’s gestational length must have contributed to the 9-month duration of pregnancy. Several hypotheses exist to explain the length of women’s pregnancies and birthing constraints. Two such hypotheses are discussed in detail within this entry: the obstetrical dilemma hypothesis and the energetics of gestation and growth hypothesis.
Hypotheses about Gestation Length
Humans have much larger brains than any other hominin. Consequently, heads of human babies are also large. Although most nonhuman primates are precocial (able to locomote and feed themselves almost immediately after birth) and humans are altricial (unable to locomote and feed themselves immediately after birth), the length of pregnancy is similar (Rosenberg and Trevathan 2002). There are several hypotheses that try to explain the timing of human gestation and birth (parturition). This entry will address the obstetrical dilemma hypothesis, which focuses on bipedalism and the narrowing of the birth canal, and the energetics of gestation and growth hypothesis, which focuses on the metabolic demands placed on the woman by pregnancy.
Obstetrical Dilemma Hypothesis
Human bipedalism and large brains can be seen as conflicting adaptations. Walking on two legs narrowed the pelvis and subsequently women’s birth canals. These changes make it harder to give birth to babies with the larger heads necessary to accommodate and foster a large brain. Early hominin skeletons, like those of Australopithecus afarensis, show signs of bipedal locomotion. Therefore, at that point human ancestors had already started to undergo skeletal changes that made bipedalism possible, including those to the pelvis. However, the brain size in proportion to the bodies in Australopithecus afarensis was still small. It is proposed that later, around the time of Homo erectus (when brain size started increasing more rapidly), the birth canal, already narrowed from bipedalism, became a limiting factor to gestation. Like humans, Homo erectus infants were likely relatively helpless at birth, as indicated by the fossil record. Thus, the obstetrics dilemma hypothesis suggests that bipedalism constrains the human gestational period such that the babies are born at this particular (and altricial) point of development because that is the point at which they are still able to pass through the birth canal. The result is an infant born with about 25% brain development of an adult human, almost half of that of nonhuman primates (Trevathan 2011). Two additional physiological adaptations allow for the birth of large-brained babies to bipedal mothers: (a) babies are born with soft cranial bones that continue to develop after parturition, and (b) women have greater hip width compared to men (who can be bipedal without having to give birth).
Energetics of Gestation and Growth Hypothesis
Although human pregnancy is in the same temporal range as that of nonhuman primates, human infants are born at a much earlier developmental stage. They require constant care because they cannot walk, eat, or protect themselves from predators, and they depend fully on their parents for survival. Having this parental role and the associated division of labor adds to both mothers’ and fathers’ costs associated with reproduction. Is bipedalism the only benefit or constraint associated with pregnancy length? If humans didn’t walk upright or have wider pelvises, would they have a pregnancy double in length? To match the gestational development of infant chimpanzees, women would have to sustain an 18–21 month pregnancy (Portmann 1990).
Pregnancy is costly to women’s metabolic processes, and greater fetal size would lead to even more energy expenditures. Thus, women may not be able to physically sustain a pregnancy necessary for the fetus to have a more developed brain. The energetics of gestation and growth (EGG) hypothesis argues that labor occurs when women are at the limit of their feasible energetic investment into the pregnancy. Dunsworth et al. (2012) examined infant body and brain size in relation to maternal body size (which is associated with the amount of metabolic resources the mothers have available) and found that brains of human infants relative to their mothers’ bodies are larger than brains of infants of nonhuman primates. Thus, although infants are born altricial and with brains at 25–30% of development, human mothers actually invest more metabolically in gestation than nonhuman primate mothers. Further, by the sixth month of pregnancy, women’s basal metabolic rate is double what it was prior to pregnancy. The EGG hypothesis suggests that pregnancy length is directly related to the balance of infant metabolism demands and maternal metabolism capabilities. It would not have been adaptive for women to further invest energy into developing an offspring at a point where their own ability to survive and pursue other important goals would be jeopardized.
As discussed in the first part of the entry, pregnancy has immense physical costs. Women’s energetic needs are doubled. Further, women become more vulnerable to pathogens because their immune system is suppressed. Especially in the latter parts of pregnancy, it becomes more difficult to move around to gather resources and escape threatening situations. Thus, women in ancestral times would have benefitted from developing different strategies for pursuing their goals when pregnant. This entry will discuss several lines of research in evolutionary psychology that examine these psychological adaptations.
During pregnancy, the mother’s immune system is suppressed in order to tolerate the genetically different embryo developing inside of her. The suppression is especially strong during the first trimester of pregnancy. Thus, women (and particularly women in the first trimester) are especially susceptible to pathogen contagion. Moreover, disease is especially pernicious at this stage because it can interrupt fetal development and result in severe issues. Therefore, women who were extra cautious in regard to what and with whom they came into contact would have had healthier offspring and increased fitness compared to women who did not regard their environment with due caution.
People’s general psychological mechanism for avoiding disease – the behavioral immune system or behavioral prophylaxis – should therefore be especially active during this vulnerable time. Indeed, women demonstrate an increase in disgust and aversion to potentially contaminated food in the first trimester of pregnancy over and above any effects of nausea and physical ailments (Fessler et al. 2005). Thus, women are more cautious of what they consume in the early stages of the pregnancy when they are most vulnerable and when it would be most problematic for them to be sick. Additionally, women’s disgust reactions to food seem to be specific to foods that are novel and/or more likely to lead to contagion and that are not nutritious dietary staples (Mckerracher et al. 2015).
Further, certain people also pose contagion-related risks. Members of dissimilar groups likely have been routinely exposed to foreign pathogens, and coming into contact with these outgroups may result in contagion. Again, women in the first trimester demonstrate an increased preference toward members of their own groups and weariness of dissimilar individuals (Navarrete et al. 2007). Thus, it seems that women have adapted to be more cautious of pathogens during pregnancy (especially the first trimester).
Motivation to Affiliate
Humans face another set of pressures associated with giving birth that is absent for monkeys and great apes (Rosenberg and Trevathan 2002). For the latter, the infant is born facing the same direction as the mother, who can then easily reach down and assist herself in the process of navigating the infant toward her chest. Human babies are born facing the opposite direction as the mother, who then cannot assist herself as the infant is emerging because that would require bending the infant backward. Coincidentally, women are unlike most mammals in that they do not typically give birth in solitude. It may be that women who were more motivated to affiliate (perhaps because of fear and anxiety) had more successful births (i.e., fewer complications for themselves and the infant) due to the assistance they were able to secure. These pressures may have resulted in an evolved preference for social birth and midwifery.
Consistent with the increased need for assistance during birth and the vulnerability to threats pregnant women face during both gestation and postpartum, research suggests that pregnant women may engage in nesting behavior as a way to protect themselves from these threats. Specifically, pregnant women prepare their physical and social spaces during pregnancy (Anderson and Rutherford 2013). Like other altricial mammals, human mothers begin preparing their physical space for the infant and do so more during the last trimester of pregnancy. Importantly, women also engage in social nesting behaviors by becoming more socially selective in their affiliation. This behavior serves multiple purposes. First, women are able to build closer networks by reinforcing relationships with their selected friends and family, who can later assist with labor and alloparenting (parenting or caretaking of the infant by anyone other than the parents). Further, by staying away from strangers and novel social environments, women are able to reduce their chances of encountering both physical and contagion threats. These nesting behaviors are not simply due to anxiety, mood, and energy levels (Anderson and Rutherford 2013).
Pregnancy requires a lot from women, and giving birth is further complicated by pelvic changes due to bipedal walking. Pregnant women have greater physiological costs and face heightened threat from disease, other groups, and complications. To mitigate these dangers, pregnant women engage in avoidant behavior of possibly threatening objects and people as well as selective affiliation to strengthen their social support.