An embryo transfer to the uterus is usually straightforward and painless: no more (or less) uncomfortable than having a Pap smear. After a speculum is placed in the vagina, a fine plastic catheter that has been loaded with the embryo is passed through the cervix into the uterus under ultrasound guidance.
Many people think that the uterus looks like it does in most diagrams (including the one on this page!), with a cave-like interior in which transferred embryos can rattle around and even fall out! In reality, the endometrial cavity is a potential space. In fact, the front and back walls of the uterus are in contact like two slices of bread with jam in the middle; the embryo is like a raspberry seed wedged in between. No matter what you do, it wont fall out!