Birth is a critical moment in the lives of two individuals, the child being born and the mother giving birth. What happens at this time may have a profound impact on subsequent development of the infant, and on the quality of the relationship between the mother and infant. Certainly, this includes mortality and morbidity related to the risks associated with childbirth. Mortality associated with birth has been high throughout human history and remains so in many parts of the world today. Thus, it is not surprising that there is a great deal of ritual surrounding childbirth designed to ensure the health of mother and infant in this perilous period. In modern countries, for those who can afford it, the danger associated with birth has led to the common practice of giving birth in hospitals surrounded by highly trained medical personnel and elaborate obstetrical technology.
Unfortunately, too much intervention in normal birth may interfere with the developing relationship between the mother and infant and, in some cases, may increase morbidity. In other words, while the decrease in mortality associated with the movement of birth from home to hospital is certainly welcomed, the impersonal and mechanical way in which births can occur in hospitals is less than satisfying to most, and may have a negative impact on the developing relationships between parents and infant.
Impact of bipedalism on birth
A common misconception is that labor and delivery are a great deal more stressful and longer in humans than in other mammals, including other primates. Because monkeys and apes, like humans, have large heads relative to their body sizes, the process of passing a neonatal head through the birth canal is not much easier for these primates than it is for humans (see Fig. 1). Exceptions to this generalization are the Great Apes (chimpanzees, gorillas, and orangutans) whose neonates are somewhat smaller relative to maternal pelvic size, leading to relatively fewer restrictions in the passage through the birth canal (Fig. 2). Additionally, the human newborn is larger overall relative to maternal bodyweight in comparison with other primates. If humans were like other primates, a mother weighing 65 kg would give birth to a baby of 2.2 kg when in fact the mean birthweight for humans is about 3.3 kg. The relatively larger size of the human fetus contributes to complexities in human deliveries.
Based on behaviors observed during labor, contractions during the birth process are painful for most monkeys and apes, and there is no evidence that the infants are born quickly or easily. But for humans, the pelvic changes resulting from the evolutionary transition from four-legged to two-legged walking have meant greater difficulty giving birth and upper limits on the size of the birth canal. These limits, associated with increase in adult brain size in the last two million years of human evolution, have meant that the human infant is much less developed at birth than our closest primate relatives. Certainly, an immature infant places further demands on the mother, in part because maintaining proximity between them is entirely the mother’s responsibility.
Bipedalism has had a number of impacts on the birth process beyond the narrowing of the birth canal and increasing immaturity of the infant. Most non-human primate infants enter and exit the birth canal in a single plane and are born facing their mothers, which facilitates reaching down and guiding the infant out of the birth canal (Fig. 3).
The human bipedal pelvis, unlike the monkey pelvis, is twisted in the middle so that the entrance and the exit of the birth canal are perpendicular to each other. This means that, for most pelvic shapes, the human fetus must negotiate a series of rotations as it works its way through the birth canal so that all maternal and fetal dimensions, including the shoulders of the fetus, line up with each other during this tight passage. Thus, the human fetus most commonly enters the birth canal facing side to side and exits facing front to back (Fig. 4). This is because the human pelvis, designed for bipedalism, has a shape that best accommodates the fetal head in a manner that results in the baby emerging from the birth canal facing toward the mother’s back. This means that the mother must reach behind her in order to guide the fetus out or she must find someone to assist her (Fig. 4).
Figure 1. Relative sizes of maternal pelvis and neonatal head for selected primate species.
Such added difficulty might explain why humans routinely seek assistance at the time of birth rather than isolation as do most other mammals, including most other primate species. Simply having someone else there to guide the baby out, to wipe the face so breathing can begin, and to keep the umbilical cord from choking the baby can significantly reduce mortality associated with birth. In fact, a survey of world cultures reveals that it is extremely unusual for a woman to give birth alone (Trevathan, 1987). Even in cultures where the ideal may be to give birth alone, such as among the !Kung of southern Africa, it rarely happens that way, especially with a first birth (Konner & Shostak, 1987).
Emotional support and the birth process
In addition to the reduced mortality and morbidity associated with having another person assist the laboring woman at the time of birth, there are clear emotional advantages to receiving support from another person rather than delivering alone. In fact, the mechanism that has been proposed to lead a woman to seek assistance at the time of birth is emotional as it is based on fear and anxiety (Rosenberg & Trevathan, 1996). In support of this are studies that consistently demonstrate the positive effects of social and emotional support at the time of birth. Furthermore, the positive effects of assistance at birth seem to persist in the first few weeks after birth, suggesting that such emotional support may have an impact on the developing mother-infant relationship. In one study, mothers who received extra emotional support at birth showed significant differences in comparison with a control group (Klaus et al., 1992). These differences included increased breastfeeding, more time spent with the infant, less anxiety, lower scores on a depression scale, higher self-esteem, and more positive feelings about partners and infants.
Figure 4. Lateral view of the human birth process showing benefits of assistance at delivery.
Figure 2. Midwife’s view of chimpanzee and human deliveries.
Figure 3. Lateral view of monkey and human passage through birth canal.
The concept of bonding
The idea that the mother-infant relationship is affected by events surrounding birth is not without controversy (Eyer, 1992). More than two decades ago, American pediatricians Marshall Klaus and John Kennell published Maternal-Infant Bonding, in which they argued, among other things, that attachment between mother and infant most optimally forms soon after birth. They referred to the process of attachment as bonding, a concept that was embraced by activists working to reform childbirth practices in US, Canadian, British, and Australian hospitals. Within a few years of publication of their topic, birth routines in some hospitals had changed to include allowing fathers and other family members to attend deliveries, minimal separation of mothers and newborn, use of birthing rooms, early breastfeeding, and minimal use of medications during labor and delivery.
‘Bonding’ became a household word, a rallying point, and, unfortunately, a source of guilt and worry for those who feared that if they were not with their infants immediately after birth (the optimal bonding period), they would not be able to bond with their children. Criticism came from feminists who argued that the concept of bonding served to reinforce stereotypes of what a ‘good mother’ is, and served to keep women out of the workplace during their childbearing years. These and other criticisms of the bonding research led most researchers and practitioners to abandon the idea that immediate postpartum bonding was part of the human behavioral repertoire, or that it was in anyway necessary for the development of attachment.
Part of the central argument about bonding in the postpartum period was the suggestion by Klaus and Kennell that there is a maternal sensitive period in the first few hours after birth during which mothers are able to bond more readily and easily with their newborns. Additionally, they proposed that human mothers exhibit species-specific behaviors at birth that facilitate bonding. These behaviors include: (1) a progression of tactile contact with the infant, beginning with fingertip exploration of extremities and face and moving on to fully embracing the infant; (2) the tendency to hold the infant on the left side of the body regardless of maternal handedness; (3) the tendency to elevate the pitch of the voice when orienting toward the infant; and (4) attempting to look into the infant’s eyes with heads in the same plane, a position known as en face.
Maternal behavior after birth
Observations of mammalian mothers interacting with their newborns reveal a number of complex and often predictable behaviors, many of which appear to fulfill fairly specific functions. These include licking or stroking the infants to establish respiration, digestion, and elimination, and to dry them so that they can maintain optimal body heat. Characteristic vocalizations are often noted that function to initiate interaction or nursing and that facilitate recognition. Most mammalian mothers position their bodies in such a way that the young can find the mammary glands. These behaviors maybe regarded as bonding mechanisms, or simply as behaviors that enhance neonatal survival. The two functions are not mutually exclusive, of course. For example, licking may serve the immediate need of stimulating respiration, but it also serves to enhance maternal recognition, and thus contributes to attachment.
The behaviors described above for human mothers have been examined almost exclusively for their effects on bonding. An enlarged perspective forces the broader question: how might they have contributed to survival in the past? For example, holding and tactile exploration of the infant may be to humans what licking is to many mammals, and thus may stimulate breathing, digestion, and thermoregulation. Accounts of left-side holding have ranged from the soothing effect of the heartbeat on the infant (not one supported by subsequent studies), the tendency for infants to turn their heads to the right, and facilitation of communication between mother and infant.
Vision is the most important sensory mechanism most primates use to get information about their environments. It is, therefore, not surprising that human mothers expend great effort looking into the eyes of their infants when they first have the opportunity to do so. Furthermore, there is evidence that human neonates can focus on objects 10-20 inches from their faces. Eye contact is one of the few behaviors under direct control of the relatively helpless human neonate. Some authors have suggested that the amount of time spent looking into an infant’s eyes (the en face position) is an indication of maternal-infant attachment. Eye contact appears to calm infants, suggesting aspects of the behavior that may have been beneficial in the past. Although olfaction may not be as important in human interaction as vision, there is evidence that the human infant can recognize the mother’s scent within several hours after birth (Porter & Winberg, 1999; see Trevathan, this volume).
Vocalizations between mothers and infants of various species serve a number of functions, including maintaining proximity, facilitating individual recognition, and initiating nursing. It has been reported that human mothers unconsciously elevate the pitch of their voices when directing their speech to or toward their infants. Additionally, the human infant seems to respond more rapidly and more intently to the higher-pitched female voice. As with en face behavior, talking to the infant in a high-pitched voice appears to have a calming effect, and was likely a common part of early mother-infant interaction among hominids in the past, as well as in the present.
There is scant evidence that contact between mothers and infants during the immediate postpartum period is necessary for survival or for adequate bond formation today. But thousands of years ago, the only infants who survived were those whose bond with their mothers began at birth and continued to an age at which food, protection, and nurturance could be derived from other sources. As with other species, we thus have a heritage of mechanisms, hormonal or otherwise, that ensures that each mother-infant dyad has optimal opportunity to initiate that bonding process, even while the infant is in utero.
Further research focusing on the relationships among mother-infant interaction, the hormones of labor and delivery, and immediate postpartum behaviors may help to elucidate the significance of contemporary environments and experiences of normal childbirth for subsequent infant development and maternal and child health. For example, there is evidence that skin-to-skin contact between mother and infant in the immediate postpartum period may have positive effects on breastfeeding success, digestion and metabolism, and in lower blood pressure and cortisol levels for the mother. These may be related to the hormones oxytocin and prolactin.
Oxytocin is involved in mother-infant attachment in many animal species, so it is likely that it plays a role in human behavior in the postpartum period and more studies of its role would be welcome. Because it is a peptide hormone, however, it is not measurable in saliva or urine and is much more difficult to assess. For this reason, many studies of its effect on maternal behavior have been correlational. Uvnas-Moberg (1999) suggests that oxytocin released at the time of birth (enhanced by estrogen, which is high at delivery) may help calm both mother and infant, reduce stress, and promote growth. If true, clinicians need to be aware of the impact of various obstetrical drugs and routines on oxytocin release. This may be particularly true for the more vulnerable preterm infant.
Far from being an isolated event, birth is just one phase in the on-going life cycle of two individuals. A broad evolutionary perspective on birth and bonding suggests that allowing women and infants to spend time together as soon as possible after birth may have a positive effect on long-term mother-infant relationships, although it is clearly not necessary for strong attachments to form. Although the idea of a sensitive period for bonding has not been supported, when we consider the intense physical and emotional experience of giving birth and the hormonal actions that accompany this process, it is hard to maintain that the first hour after birth is no different from any other hour the mother shares with the infant. If obstetrical care can complement evolved human behaviors with emotional as well as biomedical support, then mothers, fathers, infants, and society will gain.