Hi, A M,
It can be more difficult to conceive a subsequent (second, third, etc.) child if you are much older when you try to conceive the subsequent child. Unless you wait another 15+ years, based on what you've said, I don't think that you will have a harder time conceiving a second.
In my case, it was actually easier to conceive the second (live) child. I miscarried twice in my late thirties. I went through many rounds of IVF and ICSI as well as old-fashioned trying to conceive before I finally carried a child to term when I was 40 1/2. I told everyone that I was sure that I could not have any more and that my husband and I were delighted with one. To my amazement, I conceived the second naturally when I was 41 and delivered him just six weeks shy of my 42nd birthday.
Regarding determining the sex of the child, I think that there are a lot of old wives tales out there that some people still believe. Your best chance is PGD with IVF or ICSI if you produce a moderate to large number of embryos of multiple cells (six or more embryos consisting of several cells, at least seven?). Check with a reproductive endocrinologist if you REALLY want to determine the sex of your next biological child. As Joan mentioned, it's expensive. My fertility clinic charged approx. $5,000 for one attempt. I never produced enough embryos of enough cells to be eligible to even try, but I would have turned down the opportunity if I had been eligible. The sex of my child was not that important to me although I had hoped to have a girl. I ended up with two boys who are wonderful.
Best wishes,
Lynne E