This is a very intrusive procedure and all the girls are keeping quiet about it having happened? I don’t know if I buy this story about all parents not ever knowing.
Yes it is VERY intrusive, as in a speculum is placed in the girl’s vagina and the vagina is then expanded to allow access to the cervix and then a tenaculum grabs the cervix and pulls it to a position where a plastic foreign device can be inserted into the uterus via the cervical os/opening. This means an artificial object is then inside the often immature uterus and causes rejection of any embryo at the implantation stage or shortly thereafter. The foreign object causes the girls body to reject the embryo as a ‘bacterial invader’ but her body cannot flush the device out with the embryo. Since the embryo may not be ‘flushed out’ in the menses the foreign body then can cause necrotic tissue in her uterus.