Day 3 vs. day 5 vs. day 6 embryo transfer

Key takeaways

A day 3 embryo usually has about 6 to 12 cells. A day 5 or day 6 embryo has reached the blastocyst stage and has hundreds of cells.
A 2026 Cochrane review found that fresh blastocyst transfer probably leads to more live births per transfer than fresh day 3 transfer, in women with a good prognosis.
Longer culture has trade-offs: fewer embryos may be available to freeze, and some patients may have no embryo to transfer.
Day 6 blastocysts are also used for transfer. Studies suggest day 5 blastocysts have somewhat higher success rates on average, but embryo quality matters too.

What "day 3," "day 5," and "day 6" mean

The day refers to how long the embryo has grown in the lab after fertilization. According to SART, a day 3 embryo is a cleavage-stage embryo that typically has 6 to 12 cells. A blastocyst has developed for 5 to 6 days, and rarely 7. It has hundreds of cells, an inner cell mass that becomes the fetus, and a fluid-filled cavity.

The outer cells of a blastocyst go on to form the placenta, which protects and nourishes the pregnancy. Labs grade blastocyst quality based on maturity, appearance, and cell number to help decide which embryos are most likely to lead to a pregnancy (Cleveland Clinic).

What the research says about day 3 versus day 5

The best summary of the evidence is a Cochrane review updated in 2026, which pooled 36 randomized trials with 8,389 participants (Cochrane). Its main findings were:

Live birth per fresh transfer: probably higher with blastocyst transfer (moderate-certainty evidence). If 32% of women had a live birth after a fresh day 3 transfer, the review estimates that between 37% and 43% would after a fresh blastocyst transfer.
Cumulative live birth: may be higher with blastocyst transfer within 12 months of egg retrieval, counting fresh and frozen transfers from the same retrieval. This evidence is low certainty, and one large trial found little or no difference.
Miscarriage: there may be little or no difference.
Preterm birth: probably somewhat higher after blastocyst transfer, based on two studies. The review estimates a rise from 3.5% to between 3.9% and 8.5%.

These results mainly apply to women with a good prognosis. The authors note that the certainty of evidence was low for most outcomes.

The trade-offs of longer culture

Not every embryo that looks healthy on day 3 will keep developing to day 5. Growing embryos longer helps show which ones continue to develop, but it also means some people end up with fewer embryos. The same Cochrane review found that, with blastocyst culture, fewer women may have extra embryos to freeze, and more may have no embryo to transfer at all (Cochrane). For people with only a few embryos, some clinicians may discuss a day 3 transfer. This is a personal decision to make with your care team.

Day 5 versus day 6 (and day 7) blastocysts

Some embryos reach the blastocyst stage a day later than others. A 2025 meta-analysis of more than 100,000 single frozen blastocyst transfers found higher live birth rates after day 5 blastocysts than day 6 blastocysts on average (Acta Obstet Gynecol Scand). But when a poor-quality day 5 blastocyst was compared with a high-quality day 6 blastocyst, most outcomes were similar. The authors described the overall evidence as low quality.

A small number of embryos become blastocysts on day 7. A 2026 review found lower pregnancy and live birth rates with day 7 blastocysts, but noted they may still contribute to a family's chances, especially for people with fewer embryos (Human Reproduction Update). In practice, the day of development is one factor among several, alongside embryo grade and, when used, genetic testing results.

How the day of transfer relates to twins

Being able to choose among blastocysts has helped clinics transfer one embryo at a time while keeping pregnancy rates acceptable. ASRM and SART guidelines recommend transferring only one embryo when the eggs come from someone under 38 or the embryo has had genetic testing (SART). Transferring one embryo lowers the chance of twins or triplets, which carry higher risks for both parent and babies.

Questions to ask your clinician

Do you usually transfer on day 3, day 5, or day 6, and why?
Given how many eggs and embryos I have, what are the pros and cons of waiting until the blastocyst stage?
If an embryo reaches the blastocyst stage on day 6, how do you grade and prioritize it?
How many embryos do you recommend transferring for someone like me?
Will you freeze any extra embryos, and at what stage?

The bottom line

Many clinics now favor blastocyst transfer, and the evidence suggests it may improve live birth rates per transfer for many patients. It is not the right choice for everyone. Your clinician can explain how your age, number of embryos, and lab results shape the recommendation for you. This article is general education, not medical advice.