Blastocyst or day 3: when to transfer the embryo
After egg retrieval and fertilization, one of the most common questions is when the embryo will be transferred to the uterus: on day 3, or later, at the blastocyst stage (day 5 or 6).
The answer is not the same for every patient. It depends on the number and quality of embryos and on the goals of the particular cycle.
What is a blastocyst?
On day 3 after fertilization, the embryo has about 6–8 cells. If it continues to develop in the laboratory, by day 5–6 it reaches the blastocyst stage: a more complex structure with hundreds of cells, a fluid-filled cavity and two distinct cell populations — those that will form the baby and those that will form the placenta.
Reaching the blastocyst stage is itself a test of viability: many embryos that look good on day 3 arrest before becoming blastocysts.
Why does culture to day 5 help with selection?
By extending culture, we let the embryo itself “show” whether it has the potential to develop. This lets us choose, from a group of embryos, the one with the highest chance of implanting, rather than relying only on the day-3 appearance.
This selection is especially useful when we want to transfer a single embryo, reducing the risk of twins without sacrificing the chance of success.
Does blastocyst transfer increase the birth rate?
For fresh transfer, the evidence shows a higher live-birth rate with blastocyst transfer than with day-3 transfer. The most recent Cochrane review (Glujovsky et al.) confirms this, with moderate certainty.
However, when we also count the frozen transfers from the same cycle (the cumulative rate), the advantage narrows and in many studies disappears. This is because extended culture can leave fewer embryos available for freezing.
Is there a risk that no embryo reaches blastocyst?
Yes, and this is the key concern. In women with few embryos, extended culture carries the risk that no embryo reaches day 5, so the transfer is cancelled.
For that reason, in patients with few or moderate-quality embryos, day-3 transfer often remains the safer choice — for these embryos the uterus is a better environment than the laboratory.
Which patients benefit the most?
Blastocyst transfer is advantageous mainly when there are enough good embryos for selection to be meaningful; when single-embryo transfer is planned; and when preimplantation genetic testing (PGT) is performed, which requires biopsy at the blastocyst stage.
The decision, as always, is individualized by the embryologist and clinician, based on how your embryos develop in the first days.
Does blastocyst culture affect the child’s health?
The overall picture is reassuring. A small increase in monozygotic (identical) twins has been observed after blastocyst transfer, along with slight differences in gestation length in some studies. The absolute size of these risks is small.
Extended culture in a modern, accredited laboratory is considered safe practice.
Summary
Culture to the blastocyst stage is a powerful selection tool: it improves the birth rate in fresh transfer and facilitates single-embryo transfer and genetic testing. But it is not a panacea. In women with few embryos, day-3 transfer may be safer. The right timing of transfer is decided individually, based on the number and quality of your embryos.
Sources: Glujovsky et al., Cochrane Database of Systematic Reviews (blastocyst vs cleavage-stage transfer); ASRM Practice Committee, “Blastocyst culture and transfer in clinically assisted reproduction” (Committee opinion).
⚕ Medically reviewed by Prof. Stratis Kolibianakis · Thessaloniki Medical Association Reg. No. 16340
ARTICLE 3 · IVF
