Pregnancy after age 43 usually involves use of donor egg or embryos. Whereas the fertility potential of the egg rapidly declines in the late 30s to early 40s, the uterus does not "age" and success rates with donor eggs do not decline with increasing maternal age.
The primary constraint is the health of the prospective mother. Multiple studies indicate that pregnancy over age of 44 carries elevated risks of maternal and delivery complications. The overall outcomes, however, remain favorable and pregnancy in late 40s to early 50s in a healthy woman is generally considered to be a higher risk, but usually safe, pregnancy.
The decision to pursue pregnancy should be an individualized decision based on consideration of multiple factors. I usually recommend patients have a preconception medical evaluation to assess their health status, as well as a consultation with a high-risk obstetrician specialist to allow a fully informed decision about the potential risks of pregnancy. Prospective parents should also weigh considerations of parenting at an older age.