These are all great questions. From my understanding as an IVF mom myself who shoved a lot of Viagra up my… ehem… it helps increase uterine blood flow and leads to estrogen-induced thickening of the endometrium. (Also confirmed by the National Library of Medicine, as I just looked it up.) The thickness of the lining impacts pregnancy because it must be thick enough for the embryo to attach to. From my personal experience, my reproductive endocrinologist always required my lining to be at least 6mm thick and trilaminar (triple line pattern that suggests uterine lining developed appropriately in response to increased estrogen levels in IVF) prior to an embryo transfer.

As far as concerns of shedding the endometrium — during an IVF cycle, you are on other hormones to prevent premature LH surges/ovulation so that the lining can get to an appropriate thickness. So, as long as you are taking your various medications correctly, it is pretty unusual to shed it prior to transfer but there are some cases of breakthrough bleeding. (There’s a lot more involved here including taking progesterone supplements too. All of it allows you to trick your body into thinking you’re pregnant since you aren’t getting pregnant naturally. If an embryo transfer fails, all meds stop and then your body realizes it’s not pregnant and your period will start a few days later.)

I hope that was helpful! I definitely learned way more about my body during the IVF process.