Biochemical pregnancy: The fertilized egg implants on day 6-7 after fertilization, and the trophoblast cells differentiate into cytotrophoblast and syncytial trophoblast cells and start to secrete HCG around day 9. The implantation goes through three stages: localization, adhesion and implantation, and any problem in any of these stages may lead to failure of implantation. The clinical manifestation is a greater than normal serum HCG, suggesting pregnancy, but no gestational sac is seen on ultrasound examination both inside and outside the uterine cavity, and then the pregnancy state terminates spontaneously, usually referred to as a biochemical pregnancy. Here, we need to tell you whether biochemical pregnancy, failed implantation and miscarriage are the same thing? First of all, we need to clarify the concept of “implantation”. Implantation is the process in which the embryonic blastocyst in an activated state interacts with the endometrium in a receptive state, eventually leading to the establishment of a strong connection between the embryonic trophoblast and the endometrium. Failure of implantation, on the other hand, is defined as an abnormality at some point prior to the establishment of this close association between the embryo and the endometrium, resulting in the failure of the embryo to implant. A biochemical pregnancy is one in which the embryo has begun to implant, the trophoblastic ectoderm has invaded the metaplastic layer of the uterus, and the beta-HCG secreted by the embryo enters the maternal bloodstream and reaches detectable levels, except that for some reason the pregnancy does not continue later and the beta-HCG in the blood or urine is only transiently elevated. Miscarriage, on the other hand, is an event that occurs after embryo implantation and is a clinical pregnancy loss. Etiology: Abnormalities at any stage of embryonic development and the process of implantation can lead to disorders of implantation. Because of its complex etiology, it is not possible at this stage to identify the exact cause of each case of failure to conceive, which is mainly due to maternal factors, embryonic factors, and abnormalities in the dialogue between the two. Maternal factors include reduced endometrial tolerance, immunological abnormalities, hypercoagulability or embolism, etc.; 2. Decreased embryonic fertility is mainly due to embryonic chromosomal abnormalities, aneuploidy abnormalities, and poor culture environment, etc. 3. Some scholars have shown that the incidence of biochemical pregnancy is higher in those with endometrial thickness <9mm on HCG day or LH peak day, and the incidence of biochemical pregnancy is low when the endometrial morphology is trilinear endometrium. In addition, chromosomal abnormalities can also cause biochemical pregnancy. In addition to genetic factors, infections and drugs can also cause chromosomal abnormalities, and in case of miscarriage, most of them are empty gestational sacs or degenerated embryos. Moreover, the PGS technique, which has been gradually introduced in recent years, has not improved the incidence of biochemical pregnancy. Therefore, it may indicate that chromosomal abnormalities of embryos are not the main cause of biochemical pregnancy, but the endometrial condition and ovulation promotion protocol may be the main cause. Having said that, you don't have to be too nervous about biochemical pregnancy, it is still a certain percentage of spontaneous abortions, and many of them are actually biochemical pregnancies when people have menstrual-like vaginal bleeding and do not go to the hospital. What should I do if I have a biochemical pregnancy? Biochemical pregnancy is a kind of spontaneous abortion with mild symptoms, which has little impact on the endometrium and gonadal axis.