Blastocyst Transfer in IVF: Why Day 5 Embryos Can Improve Your Chances

Reviewed by Dr. Anshika Lekhi, MBBS, DGO, Fellowship in Reproductive Medicine | Infertility Expert, Gurgaon | 13+ Years

When you start an IVF cycle, the question of when to transfer, Day 3 or Day 5, comes up sooner than most people expect. Blastocyst transfer, where embryos grow to Day 5 before going into the uterus, is now the preferred approach at most experienced fertility centres, because the embryos that reach Day 5 have already shown they can keep going.

What Is a Blastocyst and How Is It Graded?

By Day 5, a fertilised egg that's developed well forms a blastocyst, a fluid-filled structure with two separate groups of cells: the inner cell mass (ICM), which becomes the baby, and the trophectoderm (TE), which becomes the placenta. Blastocysts are graded with the Gardner system on three things:

• Expansion: graded 1 to 6 (3 or above is preferred for transfer)

• ICM quality: graded A, B or C (A is compact and well defined, C has few, loose cells)

• Trophectoderm quality: graded A, B or C (A has many even cells, C has a few large ones)

A "4AA" is a fully expanded blastocyst with an excellent ICM and TE, which is the ideal. A "3BB" is good quality and transferred all the time.

Why Does Day 5 Culture Improve Selection?

The big advantage of blastocyst transfer is that the lab does some of the selecting for you. Not every fertilised egg becomes a blastocyst. Usually around 40 to 60% of fertilised embryos reach the blastocyst stage, depending on age and embryo quality. The ones that stop have ruled themselves out, so you're not transferring an embryo with little chance.

Day 3 embryos can't be judged with the same confidence. At that stage the embryo's own genes haven't fully switched on, and it's still running on factors from the egg. By Day 5, its own genes are in charge, so the blastocyst stage tells you much more about its real potential.

How Do Blastocyst Transfer Success Rates Compare to Day 3?

According to SART outcome data, blastocyst transfers consistently give higher implantation rates per embryo than Day 3 transfers. That's why single blastocyst transfer (SET) is now standard: one good blastocyst gets cumulative results similar to two Day 3 embryos, with a much lower chance of twins.

Is Blastocyst Culture Suitable for Every Patient?

No. When there are very few embryos, usually 3 or fewer, there's a real risk none will make it to Day 5 in the lab. In that case, transferring on Day 3 avoids ending up with nothing to transfer at all.

Your specialist decides based on how many embryos fertilised on Day 1 and how they develop early on. When numbers are good, going to blastocyst gives you better selection and the option to freeze extras for later. Our IVF treatment page explains how this fits into a full cycle.

What Happens to Embryos That Do Not Reach Blastocyst?

Embryos that stop growing before Day 5 have arrested, and they wouldn't have implanted if they'd been transferred earlier. At TheFertilife, we update you on embryo development at every stage so you know what's happening in the lab throughout.

Blastocyst transfer plus vitrification (flash-freezing) of spare blastocysts gives you a cumulative approach to IVF: a fresh attempt now, and frozen embryos ready for future cycles without going through stimulation again.