How Pregnancy Tests Work

How Pregnancy Tests Work

Throughout history people have tried to find a reliable way of predicting whether a woman was pregnant or not. It is known that in ancient Egypt a woman sometimes had to wear a tightly fitting metal band round her neck. If she became pregnant her neck would swell and the compression caused by the band would raise the pitch of her voice, an easy symptom to observe. Not so very long ago pregnancy was diagnosed by burning a leather bootlace: if the disgusting odours caused immediate nausea, one could assume the lady was pregnant. In the thirties the first biological pregnancy test was developed. A small sample of the woman’s urine was injected into an animal (a famous test used frogs). If the woman was pregnant, the hormones in the urine caused the frog’s skin to darken. In an alternative technique, female frogs or toads were used. The presence of pregnancy hormone in the urine caused the animals to lay eggs, thus confirming pregnancy.

Nowadays animals are not necessary any more be- cause it has become possible to perform a pregnancy test in a test tube. Such a test relies on a reaction between the pregnancy hormone human chorionic gonadotropin (HCG) in the urine and a substance in the test fluid. If there is no pregnancy hormone pre- sent – which means there is no pregnancy – a brown . – ring develops in the test tube. Such a test is very reliable. It can be performed when a period is only one week overdue. Great care should be taken however to follow the instructions precisely if you are doing your own pregnancy test using a proprietary kit, because incorrect handling (shaking, not using morning urine, performing it to early) may give misleading results.

Very recently a test has been developed in which the mix of urine and test fluid becomes another colour when pregnancy hormone is present. One of the advantages is that the test result is not liable to be influenced by shaking. Moreover it can be performed as early as a few days after the period would normally have been expected to start.

Ultrasonic examination

Clinical signs such as a missed period and typical breast changes, together with a positive pregnancy test, point fairly conclusively to pregnancy and it is rarely necessary to use imaging techniques such as ultrasound or, formerly, X-rays to confirm whether a pregnancy has been established. The ultrasonic scan is, however, the safest and most accurate method currently available for establishing the duration of the pregnancy from the size of the foetus. The uterine cavity can be imaged from the fifth week following a missed period, and the foetus within it by about the eighth week. Twins can be identified as early as about eight weeks.

Ultrasonography works by directing a beam of high-frequency sound waves onto the body. When the waves cross boundaries (interfaces) between different body tissues they are reflected back and can be observed as an image on a screen. Very low power is used and no damage to tissues of the mother or foetus has been shown to occur. During an ultrasonic examination, the movable transducer of the machine is passed back and forth over the woman’s abdomen, which first has to be oiled in order to maintain good contact between the machine and the skin. A full bladder provides a good landmark and she will probably be asked to drink some water and not to empty her bladder for a couple of hours before the test. The woman may be allowed to take a Polaroid photograph of the scan image – the first photograph of her new baby. Most women have at least one ultrasonic scan during their pregnancy, usually at about 16 weeks, but availability of ultrasound facilities varies from district to district.

X-ray examination

X-rays are no longer used routinely to diagnose pregnancy because it is known that congenital abnormalities can be caused by exposure to X-rays. It is advisable not to take risks when a woman has missed a period and the possibility of pregnancy exists. She should make sure that any doctor or dentist who recommends an X-ray knows that she is (or might be) pregnant. X-rays are potentially less harmful during late pregnancy when the baby’s organs and limbs are fully formed. In rare instances a doctor may think it necessary to X-ray the woman’s pelvis before delivery to compare the size of the birth canal with the dimensions of the baby’s head.

Foetal heartbeat

An ultrasonic scan can show the foetal heart beating as early as seven weeks after conception. Another adaptation of ultrasound, the ultrasonic detector, also known as an electronic stethoscope, enables the doctor to listen to the foetal heart from 12 weeks on. This device is particularly useful in diagnosing a twin pregnancy, the two foetal hearts being audible from just 14 weeks. An ordinary foetal stethoscope allows the heart to be heard from about 20 weeks.

A father may be able to listen to the baby’s heartbeat at one of the antenatal examinations, but with some practise he may even be able to hear it without a stethoscope, just by positioning his ear on the stomach of his partner. It is a rapid double beat, said to sound like the ticking of a watch under a pillow.