The three main stages of prenatal growth do not correspond with the three trimesters of pregnancy. The germinal stage begins at the time of conception, ending at implantation, at which point most women realize that they are pregnant since a test can pick up the pregnancy hormones put out after the blastocyst implants. During the embryonic stage, the blastocyst grows into an embryo, and the cells separate themselves into different groups to form the many systems of the body. Finally, the fetal stage is entered once the body's systems have been developed, which just continue to grow during this point. All three stages of prenatal growth either start or end within the first trimester, after which point the fetus steadily grows until it is developed enough to withstand life outside the womb.
Once a sperm successfully unites with the egg and conception occurs, the germinal stage of prenatal growth begins. The egg becomes known as a zygote once it is fertilized, and starts to move toward the uterus, after which the cells of the zygote start to divide quickly in a process called mitosis. This forms a blastocyst that is made up of three parts, one of which is the mesoderm, set to develop into the skeletal and muscle systems. The endoderm is another section, which will later become the respiratory and digestive systems, while the ectoderm is to eventually become the nervous system and skin. The germinal stage ends when the blastocyst implants into the wall of the uterus, which is usually just before most women find out about the pregnancy.
The embryonic stage typically lasts from the second week of life to the eighth, during which time both the umbilical cord and placenta are created. The cells of the embryo form the various systems, organs, and tissues of the body, resulting in the establishment of features like eyes, ears, fingers, and most other anatomical and physiological traits. By the end of the embryonic stage of prenatal growth, the necessities are formed, and just need time to grow and mature.
Weeks nine through 40 of the pregnancy are considered the fetal stage of prenatal growth, and mainly consist of organs maturing. At this point, the embryo becomes a fetus, and the spinal cord and brain are formed out of the neural tube. The rest of the pregnancy is spent adding weight and length to the fetus, and allowing his body's systems to become refined enough to survive birth and beyond.