Abortion is not allowed in the first month of pregnancy, because the presence of a gestational sac in the uterine cavity must be determined in order to perform an abortion, whether it is a medicated abortion or a manual abortion. For example, the pregnancy is usually counted from the last menstrual period, and the gestational sac is usually not present until about 42 days into the pregnancy, which allows the abortion procedure to be performed. If the menstrual cycle is regularly delayed backward, ultrasound monitoring is required at around 46 days to determine that as long as there is a gestational sac, an abortion can be performed. Similarly if the period is 7-10 days earlier every time, there may exist a pregnancy sac that can be checked at around 38 days and the procedure of abortion can be performed. In either case, it must be determined that there is a gestational sac in the uterine cavity, which must be around 1cm. In case it is too small it can cause an ineffective abortion, in case it is too large it can cause more residuals.