Friday, December 17, 2010
Human Gestation from a Medical Anthropology Perspective
Information from Ancient Bodies, Modern Lives: How Evolution Has Shaped Women's Health and Medical Anthropology: A Biocultural Approach
In the first trimester, the zygote (fertilized egg) grows and transforms into an embryo, with millions of cells differentiated into functionally distinct tissues.
It is estimated that 10 percent of recognized pregnancies end in spontaneous abortion during the first trimester. Around 50 percent of all fertilization end very early, before a woman even recognizes that she is pregnant, and these are most often due to genetic abnormalities of the zygote.
There are numerous situations in which failure of a pregnancy at this stage is a "good thing" from the perspective of evolutionary medicine.The zygote that results from the union of egg and sperm is unique, and genetically different from both the mother and father. Our immune systems are designed to deal harshly with organisms that are not familiar by damaging or rejecting them. During the luteal phase following ovulation, the mother's immune system is slightly dampened, which serves the zygote well because it is less likely to be detected and rejected.
Once the embryo is implanted into the wall of the uterus, HCG begins to be secreted and a urine test can now be used to reveal pregnancy. But a high percentage of embryos fail to implant. This is a loss for couples who are trying to conceive, but from an evolutionary perspective, it makes sense that embryos that may not have a good chance of surviving are discarded before the mother's body invests too much time and energy to gestation.
In some cases the mother may reject a fetus that is too much like her. If a woman mates with a man whose histocompatibility genes are similar to her, the resulting embryo will also be genetically similar to her. In this situation she may not recognize the embryo when it begins to implant and may not depress her immune system to prevent rejection. One suggestion is that this is an "anti-inbreeding" mechanism and it may also explain why women who have trouble conceiving with one man are easily able to get pregnant when they have a different partner.
Systems that develop during the first few weeks of pregnancy:
circulatory system (week 2)
nervous system (week 3)
limb buds, heart and most organs (week 4)
Brain and sexual development accelerate (week 5)
Nausea during early pregnancy may have evolved as a protection against toxins and other dangerous substances that could harm the developing embryo and thus may be a defense rather than a defect. This is on hypothesis of many. Morning sickness is most pronounced during the weeks when the embryo is most vulnerable. Cultural taboos are common for women, especially consumption of certain foods. Meat is a common category of forbidden food, which, as we know, meat can expose a person to dangerous contaminants.
Given that a lack of morning sickness is associated with miscarriages, it may be that nausea in early pregnancy is a signal of embryo viability and thus has selective value in itself.
By the start of the second trimester, tissue differentiation is largely complete, and the embryo is now considered a fetus. During these three months, the fetus grows rapidly in length as its skeleton grows. The mother's nutrient needs increase to support this growth.
Pregnant women will notice a lot of fetal movement early in the second trimester, beginning with frequent position changes and then smoothing out. The fetus even does some somersaults and loop-de-loops, some of which account for the umbilical cord being wrapped around the neck at birth. Movements decrease as the fetus grows bigger and movement is impeded.
The third trimester is characterized by rapid growth in weight and disposition of fat. It is also during this time that further maturation of the respiratory, gastrointestinal, and circulatory systems occurs, in preparation for the myriad changes the baby will experience after birth. Energy requirements for the mother are particularly high during this time to support the continued growth of an ever larger baby. About 80 percent of the newborn's weight is accumulated during the third trimester. Restriction of caloric intake during this time will likely reduce the weight of the newborn.
Because the fetus is entirely dependent on nutrient flow from the mother, if a mother's health is compromised in some way - due to undernutrition, infection, or stress - during pregnancy, there are likely to be consequences to birth outcome. The fetus is both vulnerable to the mother's own health problems and to some extent buffered from them.
If the fetus is gestating in a non-optimal environment, trade-offs will be made. For example, if food is restricted, available nutrients will go to the brain and not to the development of other organs. The fetal origins or fetal programming hypothesis proposes that the developing fetus uses cues to assess not only the environment of gestation but also the postnatal environment. The baby becomes programmed to expect the same conditions it experienced in utero. This can cause problems when the postnatal environment does not match the gestation environment.
These effects are also transgenerational. Cues can come not only from the mother, but also from her entire matrilineage. Because the effects are not just limited to a single pregnancy, the implication is that public health measures to improve infant birth weight should begin long before pregnancy and should not be judged as successful or failed based on data from a single generation.
Healthy pregnancies yield health children yield healthy adults, no matter what the postnatal environment is, just as unhealthy pregnancies yield unhealthy adults, even in seemingly optimal postnatal environments.