I've had ivf once and it's been successful, if i wanted to do it again, is there a treatment you can have which would determine the sex if your baby?

Yes: There is technology called 23-chromosome microarray where you can biopsy each embryo that grows to day 5 or the blastocyst stage. Not only will it give you the sex of each embryo but whether or more each of the embryos are chromosome normal. In experienced hands, the biopsy is safe and should not damage the embryo. Hope this helps.

IVF with PGD: For couples with 1 or more child in vitro fertilization with preimplantation genetic diagnosis (pgd) is a good option for family balancing. Not all fertility clinics offer pgd for non-medical reasons. I don't recommend it for a first pregnancy. Microsort, a sperm sorting technology, is no longer available in the usa. Pgd for the baby's gender is not offered in the uk, but you can come to the usa.