How Do Pregnancy Tests Work?


The diagnosis of early pregnancy is most easily and satisfactorily accomplished by performing a pregnancy test on the urine of the pregnant woman.

Many people want to know how a pregnancy test works. It is really very simple, but in order to understand it completely you will have to read this section that explains the production of chorionic gonadotrophin. This is a hormone produced by the new pregnancy even before it embeds in the lining of the uterus on the 21st day of the menstrual cycle. Its production increases day by day so that by the 28th day there is sufficient to suppress the next menstrual period, and by the 35th day or earlier there is so much that it can be detected in the urine by means of special tests. Chorionic gonadotrophin is found in its most concentrated form in the first specimen of urine passed in the early morning since nothing has been drunk for 6-8 hours.

The former tests for pregnancy involved injecting the urine into animals. The chorionic gonadotrophin caused characterstic changes in the ovaries or in other parts of the genital tract of the various animals. These changes in the laboratory animals indicated the presence of a high concentration of chorionic gonadotrophin in the injected urine and therefore meant that the urine came from a pregnant woman. These tests used to take anything from 24 hours to 7 days according to the different type of animal used.

The purely laboratory tests of today use an immunological technique. A specific antibody has been developed to chorionic gonadotrophin which makes it easily detectable in a test tube or on a simple microscope slide. These modern tests for pregnancy are as accurate as the use of laboratory animals, if not more so; they are much quicker, much more simple, much kinder and, of course, cheaper. The tests take anything from about 2 minutes to 2 hours depending on the particular type of tests being used.

A positive result indicates pregnancy in over 99 percent of instances. A negative test, however, does not necessarily mean that a pregnancy is not present because the test may have been performed too early in the pregnancy or for some particular reason the level of chorionic gonadotrophin may not yet have become sufficiently high for the test to become positive. If all the signs and symptoms of pregnancy are otherwise present, the test should be repeated after a further 7 days.

A special fraction of human chorionic gonadotrophin known as the ‘B-subunit’ can be detected in the blood of a pregnant woman as early as the 19th day of the cycle or 5 days after fertilization and 9 days before her next period is due.

See Also: Homemade Pregnancy Test


An extension of the simple ultrasonic device used for listening to the baby’s heart is the ultrasonic scanner which uses high frequency sound waves above those the human ear can detect. It may be used, without harm to mother or baby, to diagnose a pregnancy as early as 5 or 6 weeks from the last period. In addition, ultrasonic scans are used extensively in later pregnancy to study the growth of the baby, diagnose twins and find out to which part of the uterus the placenta has become attached.

The fetal heart

The fetal heart can normally be heard by the stethoscope at about the 24th week of pregnancy. The more modern method of listening to the baby’s heart is by means of ultrasonics, using the Doptone or the Sonicaid apparatus which can detect the fetal heart at the 12th week of pregnancy and frequently as early as the 10th week. Ultrasound has replaced the use of X-rays as a means of determining the presence of a fetus in the uterus in early pregnancy, and furthermore it does not carry the disadvantages of X-rays. Ultrasonic scans will demonstrate movement of the fetal heart as early as the end of the 6th week of pregnancy.

Clinical examination

The diagnosis of pregnancy before the end of the 8th week by simple clinical examination alone is extremely difficult. The most certain method of establishing an early diagnosis is by means of a pregnancy test which will be positive shortly after the time of the first day of the missed period.

Most doctors get a pretty shrewd idea that a pregnancy is present from the history given by the patient. If all the usual symptoms of early pregnancy are present, then it is fairly good presumptive evidence that the woman is pregnant. Most women, however, want a definitive answer which it is impossible to give based on the symptoms alone. Examination of the patient herself may show the characteristic breast changes. Vaginal or internal examination will not show any significant changes until about 6 weeks after the first day of the last period. These changes, when they do appear, will include a bluish or violet discoloration of the vagina, a softening of the cervix, a slight enlargement and softening of the uterus, together with a palpable pulsation of the uterine artery which cannot be detected in the non-pregnant state. All these physical signs are rather uncertain and if a definite answer is required then the pregnancy will usually have to be confirmed by performing a pregnancy test upon the urine. A special blood test in very early pregnancy or ultrasonic scan at 6 weeks will also confirm pregnancy.

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About Adler Moris

Professionally, I am writer, blogger and IM expert. But, my passion to keep me fit makes me able to share best what I've got about the perfect health. I love my work the most but my beautiful wife took first place after marriage :). However, I am huge fan of physical activities, meeting new people, reading and writing news to take my expertise to next level.