pregnancy tests

views updated

pregnancy tests The cardinal feature of pregnancy is of course the cessation of menstruation. But some women have irregular menstrual cycles, and there are many other possible causes of amenorrhoea. The early signs of breast changes, urinary symptoms, and ‘morning sickness’ can confirm a suspicion within a few weeks of conception, but there may be a more urgent desire or pressure to be certain. For a variety of social, cultural, and sometimes legal reasons, there has no doubt been a very long history of tests for pregnancy. One such is recorded in the Ebers papyrus (about 1350 bce): ‘a water melon is pounded, mixed with the milk of a woman who has borne a son, and is given to the patient to drink. If she vomits she is pregnant, if she has only flatulence she will never bear again.’ Modern pregnancy tests are based on the detection of human chorionic gonadotropin (HCG), a hormone unique to pregnancy that starts to be produced by the embedding ‘chorionic villi’ of the embryo as soon as it becomes implanted in the uterus. The hormone can therefore reach the mother's bloodstream within days of conception, and is excreted in the urine. Tests for its presence by immunoassay on samples of urine or of blood are nowadays very sensitive and rapid.

Earlier methods of bio-assay for identifying HCG in the mother's urine depended on the use of mice or frogs. In the pregnant woman, the hormone acts on the ovaries so as to maintain the secretion of the ovarian hormones that in turn make possible the continuation of pregnancy; without them, there would be menstruation. If injected into animals, gonadotrophin stimulates the ovaries and ovulation occurs.

Ascheim and Zondek were the first to show, in Berlin in 1927, that pregnancy could be confirmed even before the first missed period, by injection of an extract of the mother's urine into immature female mice. The ovaries were stimulated, causing enlargement with development of eggs that could be seen in the abdomen when the animal was killed after a suitable interval. In 1933, a successful pregnancy test was achieved by Shapiro and Zwarenstein in Cape Town, using a particular variety of frog (Xenopus), which had been shown to respond to injection of gonadotrophins by ejecting visible eggs from the body. This Xenopus test was refined over succeeding years and in the 1940s the Cape Town laboratory reported >98% correct results. Reporting the history of the test in 1985, Zwarenstein put on record a letter from a family doctor who wrote: ‘Thank you for your report on the pregnancy test on Mrs X. You may be interested to know that of one GP of many years' standing, one specialist gynaecologist and one frog, only the frog was correct.’ The bio-assay methods continued until the late 1960s, when immunoassay took over.

Sheila Jennett, and E. M. Tansey

See also antenatal development; placenta; pregnancy; sex hormones.