Letrozole

Hi ladies! I am 38 (39 in March) I have one 17 year old daughter. My partner and I decided to start TTC back in the fall. After a few months with no luck we met with a fertility specialist. All of the test run on myself and my partner indicated that there was no reason that we wouldn't conceive naturally. After a few months of timed intercourse my dr put me on 5mg of Letrozole for 5 days. I had no side effects. I took it at night. I produced 3 large follicles, confirmed ovulation but unfortunately just took a pregnancy test today, 4 days before the 1st day of my missed period due date and it came out negative. Any feedback is greatly appreciated.