IVF Pregnancy. Instead of medicines.

I am taking IVF treatment as my both tubes get infected during last 2 pregnancies which was ecopic, so it's removed Now my concern is I find problem for endometrium thickness.. When I take medicines given by Dr for ET, it doesn't help to increase, it's comes around 5-6.. On other hand, naturally (withoutET med) my ET comes around 9mm. Now, my IVF Dr says we need ET which is only developed by medicines n not by naturally even its 9mm. If Endometrium thikens wid the help of medicines then only we go ahead for next process. Naturally thickened endometrium will not help. This month my ET was 9 mm wid 3 layers but still Dr said take med for ET n come in next month. From last 1.5 years m taking treatment. No other prob. Plz guide me, is it necessary to develop ET only through medicine for IVF, coz naturally ET comes to good number instead of medicines.