First time delivery, big baby, prolonged labour, if you need ventouse of forceps to deliver the baby, in case the heartbeat of the baby is slowing down and various such situations.
It is safer to allow the doctor to do an episiotomy than to try delivery without the cut especially if the perineum is not stretched. If the perineum is not well stretched, then the muscles may tear leading to a tear through important structures like the anal sphincter and that will not only be difficult to repair but also affect function in the future.
The episiotomy is safe and once stitched and healed, will not have any long term implications. In some cases, there may be swelling, excess bleeding, infection and delayed healing. This may happen in anaemia, diabetics, instrumental delivery, and where vaginal manipulation may have been done eg manual removal of placenta.
After the delivery you must look after the episiotomy. You must keep the area clean and dry. You can apply a simple anti-septic cream in the area. Hot or cold fomentation and sitz bath can help reduce the pain and the swelling in the first few days.
You must avoid sex till the stitches have healed and ideally till the bleeding has stopped. It is advisable to avoid tampons in the post delivery period.
Episiotomy is safe, helps to deliver the baby and in most cases, heals well.