There are no necessary analyzes at the fourth week of pregnancy yet, but if you cannot wait to find out whether you are pregnant or not, you can take a blood test for hCG and progesterone.
At the 4th week of pregnancy blood test for HCG gives a quite clear positive response, as far as there is sufficient amount of progesterone in the blood already. The result of the analysis of blood for progesterone will also show elevated level of this hormone, which may indicate the presence of pregnancy.
At the 4th week of pregnancy you can do an ultrasound examination, which can identify the corpus luteum of pregnancy – a temporary endocrine gland that is formed after ovulation and produces the hormone progesterone.
If the fertilization has occurred, yellow body (now it is called the corpus luteum graviditatis) remains active for 10-12 weeks under the influence of the human chorionic hormone, producing sufficient amount of progesterone for the development and preservation of pregnancy, which stimulates growth of the endometrium and prevents the release of new eggs and menstruation. Yellow body is preserved until the placenta is not in a position to produce its own estrogen and progesterone.
Although modern ultrasound machines are able to determine that the fetus was already fixed on the wall of the uterus at the 4th week of pregnancy, similar researches at such an early period are held very rarely, because the woman may not know yet or just starts to think about possible pregnancy.