Negative Pregnancy Test

My partner and I have been struggling to get pregnant. Today makes 16 past my period, without any signs of approaching period or pregnancy symptoms, with two negative pregnancy test, to say I am disappointed would be stating the obvious. I am 38 years old and already have a 12 year old, so this is my last try. At my last gyno visit I was told that my uterus and ovaries are more than in a good place for pregnancy at my age, but my husband does have a low sperm count at the age of 55. We had seriously considered going through the fertility process, but wanted to give it one more go naturally. I am so confused as to why I am this late for my period, but I guess it's back to the drawing board, and IVF may look like our best option. It's just so frustrating :-(