At 4 weeks of pregnancy, the fetus has just been laid and is still relatively small, the amniotic cavity has just been formed, and the fetus is still an embryo consisting of two layers of tissue, the traces of which are not yet visible on ultrasound. A woman’s pregnancy is counted from her last menstrual period. Pregnant women with regular menstruation usually ovulate 14 days before their next menstruation. If the sperm and egg combine to form a fertilized egg at this time, it will be deposited in the uterine cavity after 6-7 days. At 4 weeks of pregnancy, the embryo has been developing in the uterine cavity for about a week, and the fetus is still very small and still in the stage of differentiation. The embryo at 4 weeks of pregnancy is only made up of two layers of cellular tissue, and later on the fetus’s organs, body, limbs, etc. will develop from these differentiations. Pregnant women should pay attention to avoiding exposure to harmful substances and should not blindly take drugs, so as not to affect the differentiation of the embryo and lead to fetal deformities.