During the first weeks after fertilization, the future mom still does not feel the effects of pregnancy, but that does not mean that your baby is not developing, on the contrary, it keeps growing every second. As the woman's body is preparing to host a new being, he also suffers several transformations. The first is the absence of menstruation, which occurs through the production of certain hormones that prevent the shedding of the endometrium. From the fourth week after fertilization, the embryo begins producing the hormone human chorionic gonadotropin beta or beta-HCG, acronym, which causes symptoms such as nausea, fatigue and pain in the breasts. During this period, pregnancy tests sold in pharmacies may be unable to detect the hormone present in the urine, but a blood test can detect pregnancy certainly.