Toxic chemicals, illegal drugs, alcohol, some medications, tobacco smoke, X- ray treatment, and STDs all contribute to birth defects and should be avoided. Embryonic development lasts from fertilization to the end of the second month of gestation, at which time all organ systems have formed. The zygote undergoes the morula stage, and the blastocyst implants in the uterine lining by the end of the second week. The inner cell mass is present, and extraembryonic membranes are forming. At the end of the first month, organs are developed and the placenta is forming.
Limb buds are present, and eyes, ears, and a nose appear.
During the first two months, the mother may experience nausea, breast tenderness, fatigue, and other symptoms. At the end of two months, the embryonic stage is over. Fetal development extends from the third to the ninth month. Bone is replacing cartilage. During this time, it becomes possible to distinguish males from females.