biology and evolutionary biology. Biodemographers do research on demographic outcomes such as conception, spontaneous abortion, births, marriage, divorce, menarche, menopause, aging, and mortality. Biodemographers use mathematical models, statistical estimates and biomarkers to analyze the demographic data. The field of biodemography often explores the scientific questions associated with fertility and mortality across cultures, the determinants of reproductive senescence, mortality and sex differences, low fertility in humans, and longer post-reproductive lifespan in women. Key topics in reproductive ecology Gestation In human reproductive ecology, the study of pregnancy is primarily focused variation in pregnancy and on rates of pregnancy loss. Variation in pregnancy Pregnancy varies person-to-person and across cultural and socioeconomic lines. Human gestation is between 30 and 40 weeks long. The dynamic between the mother and the fetus is one of conflict: it is in the best interest of the fetus to gestate as long as possible to continue receiving the nutritional and developmental benefits of being physically attached to the mother. For the mother, however, pregnancy is a highly demanding and risky time. Earlier births avoid complications in the birth of a too-large infant. The length of the pregnancy is a compromise between these two demands, and is influenced by factors such as socioeconomic status, health, and fetal development. Women of lower socioeconomic status have been shown to deliver their babies earlier on average than women of higher socioeconomic status. Research has also shown that stress, especially during early pregnancy, can cause shorter gestation length and increase premature births. Pregnancy loss The rate of embryo loss changes throughout pregnancy. Pre-implantation in the uterine wall, rate of loss is undetectable as the hCG hormone is not secreted until implantation. There is no current way to detect pregnancy or pregnancy loss at this stage. Post-implantation, rate of loss is highest in the first trimester of a pregnancy. The chance of pregnancy loss lowers the further into gestation a woman is. Pregnancies may be unsuccessful for multiple reasons. The maternal immune system, though suppressed during ovulation, views the fertilized egg as a foreign body and will attack it. Defective embryos may also be spontaneously aborted, or miscarried, whether due to chromosomal abnormality or developmental defects. Endometrial or placental development issues may also cause a pregnancy to fail. Additionally, the frequency of spontaneous abortion increases with the mother's age. Older mothers have a higher rate of genetic abnormalities that can trigger pregnancy loss. Because human