Why do men have only two levels of ordination while women have to go through three levels of ordination?
There are 5 different types of ordination, 2 for men and 3 for women. Samaneri (female novice) is a lower ordination (bappajja) with 10 precepts. One who is entitled to this ordination must be old enough to be able to drive away crows off the paddy field. Later on it was fixed at a minimal age of seven. Older men sometimes also receive only lower ordination.
One who can apply for higher ordination (upasampada) must meet the minimum age requirement of 20 and have a normal physical condition. A married woman may be ordained at the age of 12 with permission from her spouse.
For Sikkhamana, a training period was instituted later. There was a particular case of a married woman who asked for ordination without knowing that she had conceived. Her pregnancy came to light only after ordination. Hence a rule was laid down and women have to go through Sikkhamana training for two years.
During this time a sikkhamana observes 6 anudharmas which are the first six precepts in the 10 precepts for novices. However, a sikkhamana is considered having a training level higher than novice even though the latter received only six precepts. This implies that a sikkhamana is in fact a person in a preparatory stage to receive higher ordination, and during the two years training, should she transgress any one of the precepts she has to start all over again. It is required that she must meet the requirement of two continuous years of training without transgression.