After the sperm and egg are combined, it usually takes about 6-7 days for the sperm and egg to implant, and the implantation time varies from woman to woman depending on her physical condition. Men and women have intercourse after the sperm and egg in the fallopian tube, combined to form a fertilized egg, the fertilized egg with the help of the fallopian tube peristalsis, fallopian tube epithelial fibers to promote the direction of the uterine cavity to move. The fertilized egg enters the uterine cavity around the 4th day after fertilization, and on the 6th-7th day, the fertilized egg will find a suitable location in the uterine cavity to settle down. Each woman’s physical condition is different, some women ovulate later and the fertilized egg may be delayed, for example, about 7-10 days, before it is successfully deposited. The fertilized egg is still small in the uterus and cannot be seen by ultrasound. After 15 days, the gestational sac will grow to 1cm in size and may be visible by ultrasound.