Pregnancy is often compared to a semi-compatible transplant. The embryo is formed by the fusion of a maternal cell (the egg) and a paternal cell (the sperm). Normally, a foreign cell would be recognized and destroyed by the immune system. However, implantation is a highly complex process that allows the maternal body to "accept" the embryo for the duration of pregnancy through a combination of immune and vascular adaptations.  

For successful implantation and development, two key conditions must be met:  

1. A genetically healthy embryo (free from major chromosomal abnormalities).  

2. A well-prepared endometrium, meaning that it is hormonally, immunologically, and vascularly receptive.  

If these conditions are not met, implantation may fail, leading to either:  

- No pregnancy (the embryo does not attach to the uterine lining).  

- Early miscarriage, most often occurring in the first trimester.