IVF, ICSI and Preimplantation Genetic Testing (PGT)

The term "third-generation IVF" is widely used in Chinese-language fertility discussions. It refers to preimplantation genetic testing: embryos are biopsied and genetically tested before transfer. This page explains what each technique actually involves, in the order a patient experiences it.

How the techniques differ

Conventional IVF	Eggs and sperm are placed together in a culture dish and fertilisation occurs without micromanipulation.
ICSI	A single sperm is selected and injected directly into a mature egg. Used where sperm motility or count is low, where previous fertilisation failed, or where the egg's zona pellucida presents a barrier. In Chinese usage this is often called second-generation IVF.
PGT (third generation)	Embryos are cultured to the blastocyst stage; a few trophectoderm cells are biopsied and genetically tested; an embryo is then selected for transfer. PGT is a selection tool, not a treatment that improves an embryo.

The three types of PGT

PGT-A — aneuploidy screening

PGT-A checks whether an embryo has the expected number of chromosomes. A human cell normally carries 23 pairs, 46 in total; an embryo with a missing or extra chromosome is described as aneuploid. Most aneuploid embryos fail to implant or miscarry, and the probability of aneuploidy rises with maternal age. PGT-A is most often discussed for patients of advanced maternal age, those with recurrent implantation failure, or those with recurrent pregnancy loss. It does not detect all genetic conditions and it is not a guarantee of a healthy pregnancy.

PGT-M — single-gene disorders

PGT-M is used where one or both parents carry a known pathogenic variant, such as thalassaemia, spinal muscular atrophy, cystic fibrosis or Huntington's disease. Testing targets that specific variant so that embryos carrying it can be identified. It requires prior confirmation of the familial variant and usually a bespoke test set-up.

PGT-SR — structural rearrangements

PGT-SR is for parents carrying a structural chromosomal rearrangement, most commonly a balanced translocation. Carriers are usually healthy but produce a high proportion of embryos with unbalanced genetic material, which raises miscarriage risk. Conventional approaches cannot always distinguish a fully normal embryo from a carrier embryo; we discuss the limits of discrimination with each patient before testing.

Step by step

1. Stimulation — daily injectable medication for roughly 10–14 days, monitored by ultrasound and hormone measurements.
2. Trigger and retrieval — a trigger injection matures the eggs; retrieval is a short procedure under sedation.
3. Fertilisation — conventional IVF or ICSI depending on the indication.
4. Blastocyst culture — embryos are cultured for five to six days to the blastocyst stage.
5. Biopsy — where PGT is used, 3–5 trophectoderm cells are taken. The procedure is performed by the embryology team.
6. Testing — results are typically available in 5–7 days, depending on the platform used.
7. Frozen embryo transfer — because testing takes time, PGT cycles are usually managed as freeze-all, with transfer in a later prepared cycle.

What PGT does not do

• It does not improve embryo quality; it identifies which embryos to transfer.
• It does not screen for every condition. PGT-A looks at chromosome number, not single-gene disorders or most structural problems.
• A normal result does not guarantee a live birth, and it does not remove the need for prenatal screening and monitoring.
• It may leave a cycle with no transferable embryo, particularly where few embryos reach the biopsy stage.