Pregnancy: Introduction
NB: See learning outcomes for this tutorial mapped to competencies, a PDF of the whole text, and a one-page summary
☞ Why this subject matters…
The number of women needing to
take medicines during pregnancy is increasing, in part due to women becoming
pregnant at an older age, and requiring treatment for pre-existing medical
problems, and/or suffering obstetric complications. It is essential that women can make informed
decisions about whether to use a medicine during their pregnancy. Therefore you
need to be familiar with the stages of development of the embryo and the fetus,
the potential harm that medicines can pose, and strategies to reduce any
risk(s).
Teratogenicity
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The incidence of major congenital malformations in the UK general population is estimated to be between 2-3%. A high proportion of these malformations are of unknown aetiology. Some may be due to the mother's health during pregnancy or her lifestyle; only 1-2% are thought to be due to drugs.
There are diagrams illustrating when teratogens might be more likely to adversely affect specific aspects of fetal development, including this example.
The embryo is most vulnerable to teratogens during the embryonic phase, from days 18 to 55, when the cells differentiate and the major organs are formed. If differentiated cells are damaged they are unlikely to be replaced resulting in permanent malformations. During the fetal period, from day 56 until birth, organs such as the cerebral cortex and the renal glomeruli continue to develop and remain particularly susceptible to damage. Functional abnormalities such as deafness may also occur.
As it may be difficult to know exactly what date ovulation and conception happens, the stage of a woman’s pregnancy is counted from the first day of her last menstrual period (LMP). For example, a woman who misses her period, finds out she is pregnant, and the first day of her LMP was four weeks ago, is described as ‘4 weeks pregnant’. This is despite conception only happening about 2 weeks ago.
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Teratogenicity is not the only risk posed by medicines. They can, for example, trigger spontaneous miscarriage (also known as spontaneous abortion). The background incidence of spontaneous miscarriage is about 10-20% of all pregnancies.