Hi Jenny

I have never been on metformin but was on clomid for 2 cycles.The reason they do the blood test at day 21 is because metformin and clomid as supposed to make your cycle roughly a 28 day cycle, so they figure if you ovulate after day 21 theres really no chance of getting pregnant that close to the end of your cycle.(thats what gyno told me).

There are heaps of other girls here who have been on metformin who'll be able to answer your question more directly though.

Goodluck................