In a yoga practice there are certain poses that should be avoided during menstruation. The main type of poses are inversions.
The reasoning for this is that when we practice inversions when menstruating it goes against the natural flow. It could stop or disturb the menstruation and lead to other reproductive problems later on. Another reason is that during inversions the uterus is pulled towards the head and causes the broad ligaments to be over stretched which cause partial collapse of the veins, leaving open arteries to continue pumping blood. This can lead to vascular congestion and increased menstrual bleeding.
More poses to avoid
particularly strong backbends, twists, arm balances and standing positions that put a lot of stress on the abdominal and pelvic region. The reasoning for this is if the pelvic region is causing spasm and pain why cause more cramping to the area.
Many positions can still be practiced and explored. It's all about listening to our bodies needs and accepting what we are capable of, and of course, what we need to take it easy on!