Epidemiologic measures of the course and outcome of pregnancy.

Successful reproduction, the spectrum of events leading from conception to birth of a healthy infant, is both biologically and epidemiologically complex. Problems that arise during the course of the reproductive process define the adverse outcomes in epidemiologic studies of pregnancy. A simplified time line for the process leading from conception to birth is shown in figure 006F1, along with an indication of approximately when critical events occur. With the focus first on the desired or “normal” outcomes, conception results from a viable sperm’s reaching the ovum and progressing to implantation. Normal development over the first weeks of life depends on differentiation and migration of cells, events that must follow precise timing, leading to formation of organ systems and subsequent fetal growth and development. Reproductive epidemiology encompasses the entire scope of these events, often extending backward to the determinants of conception (e.g., semen quality, menstrual cycles) and forward to postnatal health and development, often through infancy, childhood, and puberty, and occasionally to adulthood. For the purposes of this paper, our focus is on the narrower time frame of conception to birth, with full recognition that the boundaries are arbitrary and sometimes ambiguous.