Why Are Pregnant Women Considered a Population at Risk?
Pregnant women are defined as a population at risk mainly because of the developing fetus. The fetus growing in the mother's uterus receives its nourishment and oxygen from her, and it is sensitive to various environmental agents to which the mother may be exposed during the pregnancy.
Environmental agents, substances or diseases that could affect fetal development in the uterus are termed teratogens. Only a few dozen agents are known to be teratogenic, among them ionizing radiation, certain viral infections (such as rubella and cytomegalovirus infection), metabolic imbalance in the mother (e.g. diabetes), certain drugs (e.g. isotretinoin, used for treating acne), environmental chemicals (e.g. organic mercury, pesticides and industrial solvents), hypoxia (deficient oxygen supply), hyperthermia (heat stroke), etc.
From fertilization through to delivery, the fetus goes through a number of developmental stages. The stage of fetal development influences its susceptibility to certain teratogens. Different organs may be affected at different stages of the pregnancy, and each organ has a critical period at which it may be harmed. The fetus is particularly sensitive to harmful substances during the first half of the pregnancy, which includes the preimplantation stage and organogenesis, as described below.
Fetal development is divided into three stages:
- Preimplantation period – lasting about two weeks, from fertilization up to implantation of the fertilized ovum in the uterus. During this stage the fertilized ovum is still composed of stem cells with the capacity to develop into any cell in any body system. Thus, if the agent affects numerous cells, the fertilized ovum will not continue to develop, resulting in a spontaneous abortion. In most cases, when a large proportion of cells remain unharmed, the fertilized ovum will overcome the damage and continue to develop.
- Organogenesis – this stage stretches from day 18 to day 60 after fertilization. During this stage the fetal cells divide and are transformed into cells characteristic of specific systems of the body (a process known as differentiation). The main body organs are formed and increase in size. Exposure to teratogens during this stage may interfere with the normal development of organs and systems, and cause marked defects.
- Fetal period – At this stage the formed organs increase in size. During the late fetal period (starting from week 25) the risk of a harmful effect of teratogens on fetal growth or on sensitive organs, such as the central nervous system, is reduced. However, sensitivity to harmful environmental influences is still high, but generally inversely related to the stage of exposure. This principle is elaborated in the section of Populations at Risk that deals with children and adolescents.
It is thus clear that exposure of the fetus to teratogens at critical stages of development may cause long-term damage. The degree of exposure to the agent, the dose of exposure, the developmental stage of the fetus at the time of exposure, and the metabolism of the mother and the fetus all play a role in determining the effect of the teratogen on the fetus,
References
- Gardella JR, Hill JA 3rd. Environmental toxins associated with recurrent pregnancy loss. Semin Reprod Med. 2000;18:407-424.
- Kheifets L, Repacholi M, Saunders R, van Deventer E. The sensitivity of children to electromagnetic fields. Pediatrics. 2005; 116:e303-313
