The number to be transferred depends on the number available, the age of the woman and other health and diagnostic factors. In countries such as Canada, the UK, Australia and New Zealand, a maximum of two embryos are transferred except in unusual circumstances. In the UK and according to HFEA regulations, a woman over 40 may have up to three embryos transferred, whereas in the US, there is no legal limit on the number of embryos which may be transferred, although medical associations have provided practice guidelines. Most clinics and country regulatory bodies seek to minimise the risk of multiple pregnancy, as it is not uncommon for multiple embryos to implant if multiple embryos are transferred. Embryos are transferred to the patient's uterus through a thin, plastic catheter, which goes through her vagina and cervix. Several embryos may be passed into the uterus to improve chances of implantation and pregnancy.

Secondary infertility (SI) is defined by doctors as the inability to conceive or carry to term a second or subsequent child. You may not have heard of it but you probably soon will, because it's on the increase. A US study revealed that, in 1995, 1.8 million women suffered from secondary infertility; in 2006, it was 3.3 million. SI now accounts for six out of 10 infertility cases.

At RMA, once the embryos reach the blastocyst stage, they are tested through a process called Preimplantation Genetic Testing for Aneuploidy (PGT-A), which lets doctors know which embryos have a normal number of chromosomes. While genetically normal embryos are much more likely to lead to pregnancy and healthy babies, the transfer of abnormal embryos will either result in no pregnancy, miscarriage, or an affected baby. While testing is occurring on a small part of the embryos, the embryos themselves are frozen, awaiting a receptive uterus. A large, prospective study performed recently at RMA confirmed that performing an embryo biopsy does not harm the embryo and does not decrease the likelihood of implantation.
Luteal support is the administration of medication, generally progesterone, progestins, hCG, or GnRH agonists, and often accompanied by estradiol, to increase the success rate of implantation and early embryogenesis, thereby complementing and/or supporting the function of the corpus luteum. A Cochrane review found that hCG or progesterone given during the luteal phase may be associated with higher rates of live birth or ongoing pregnancy, but that the evidence is not conclusive.[79] Co-treatment with GnRH agonists appears to improve outcomes,[79] by a live birth rate RD of +16% (95% confidence interval +10 to +22%).[80] On the other hand, growth hormone or aspirin as adjunctive medication in IVF have no evidence of overall benefit.[30]