A pregnancy is for 4 weeks after your period to be correct... but here's some info on a pregnancy test
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.
Home pregnancy tests are great to use because they can be done at home, they are usually low in cost (anywhere from $7.99 to $19.99), private, they give a fast result, and are easy to use. However, if not done correctly or taken too early, the result can be inaccurate.
If you get a negative result and still have symptoms of pregnancy (missed period, nausea, breast tenderness and fatigue), wait a week and take another test or contact your doctor so you can have a blood test done.
Advantages of having a blood test done:
* Can detect a pregnancy earlier than a urine test at about 7-12 days from possible conception ( but if a negative result is received, a test should be repeated if a period is missed.)
* Can measure the concentration of hCG hormone in your blood (this is useful information for your healthcare provider in tracking certain problems in pregnancy)