The first week of menstruation can turn into the first week of pregnancy if fertilization of the egg and implantation of the fetal egg occur in this menstrual cycle. If pregnancy does not occur in this cycle, the countdown starts again in the next cycle.
Obstetric 1 week of pregnancy begins on the 1st day of the last menstruation, and 1 embryonic week is counted from the moment the egg is fertilized. That is, 1 week of pregnancy in obstetric terms precedes ovulation and fertilization. Conception usually occurs between 2 and 3 obstetric weeks.