The normal pregnancy is the union of sperm and egg, generally seven days after the union of sperm and egg will secrete chorionic gonadotropin, after another seven days, we can use the early pregnancy test paper to detect, often sperm and egg union, secretion, but must be combined into a fertilized egg, fertilized eggs have to return to the uterus to bed, biochemical pregnancy is the union, but did not return to the uterus to bed, this is called biochemical pregnancy. The following are the distinctive features of biochemical pregnancy: 1, urine test is weakly positive, but it is very difficult to reach the positive, much less strong positive. 2, ultrasound intrauterine can not see the gestational sac. 3, low blood hcg, can only indicate that the pregnancy, but not whether the successful implantation. 4, usually will not exceed 50 days will be spontaneous abortion. The main reason for biochemical pregnancy is poor embryonic development and chromosomal abnormalities. This is more common in some women who are undergoing IVF. However, most women do not go to the hospital for examination and they do not care about it, so they ignore it as a delayed menstruation, but in fact it is already a spontaneous miscarriage. Do not be alarmed if a woman of age encounters such a situation, as it is the result of natural elimination and usually does not affect the next pregnancy. If this happens repeatedly, it is necessary to go to the hospital for further examination. Check both sides for chromosomes, teratogens, anticardiolipin antibodies, antinuclear antibodies, etc.