What Is the Difference Between a Day 5 and Day 6 Embryo?

Short answer: A day 5 embryo and a day 6 embryo are usually both blastocysts. The difference is how quickly they reached that stage after fertilization. A day 5 blastocyst developed sooner; a day 6 blastocyst needed one additional day. Day 6 does not automatically mean poor quality, and transfer decisions should also consider chromosome screening, embryo grade, egg age, and the clinic laboratory's experience.

What is a blastocyst?

A blastocyst is an embryo at an advanced stage that typically begins on day 5 or day 6 after fertilization, although some embryos reach it on day 7. It contains a fluid-filled cavity, an inner cell mass that can develop into the fetus, and an outer cell layer that contributes to the placenta.

IVF laboratories monitor embryos as they develop. Once an embryo reaches an appropriate blastocyst stage, it may be frozen, biopsied for preimplantation genetic testing, or selected for transfer.

Why do some embryos become blastocysts on day 5 and others on day 6?

Embryos do not all develop at exactly the same speed. Small differences in cell division, egg and sperm factors, and laboratory conditions can affect when an embryo reaches the blastocyst stage. The day of blastulation is useful information, but it does not determine an embryo's potential by itself.

A day 6 embryo can have strong morphology, be euploid on PGT-A, and result in a healthy live birth. It should not be dismissed simply because it developed one day later.

Do day 5 embryos have better success rates?

Many studies have found an average advantage for day 5 blastocysts over day 6 blastocysts when large groups are compared. However, the difference for an individual embryo depends on whether it was tested with PGT-A, its morphology, the age of the egg source, freezing methods, and the laboratory's protocols.

When both embryos are euploid, the difference may be smaller than it is among untested embryos. Clinics may still rank embryos differently, so intended parents should ask the reproductive endocrinologist or embryologist how their laboratory uses developmental day in its transfer order.

Which matters more: the day, embryo grade, or PGT-A?

Each measure provides different information:

• Developmental day describes how quickly the embryo reached the blastocyst stage.
• Morphology grade describes how the embryo looks under the microscope, including expansion, the inner cell mass, and the trophectoderm.
• PGT-A estimates whether the sampled cells have the expected number of chromosomes. It does not test every cell and cannot guarantee implantation or a healthy baby.

Clinics combine these factors with the patient's history. There is no universal rule that every day 5 embryo must rank above every day 6 embryo. For example, a euploid day 6 embryo may be prioritized over an untested day 5 embryo or one with an abnormal PGT-A result.

Is it normal to freeze an embryo on day 6?

Yes. Freezing blastocysts on day 6 is routine in IVF laboratories. Professional guidance notes that current practice commonly allows cryopreservation on day 5 or day 6, which can make more potentially usable embryos available over the full treatment journey.

For a frozen embryo transfer, the clinic coordinates endometrial preparation and transfer timing with the embryo stage and its protocol. The label "day 6" alone does not mean the embryo is unsuitable for transfer.

What about a day 7 embryo?

A smaller number of embryos reach the blastocyst stage on day 7. Some clinics continue culture and will biopsy or freeze these embryos when they meet laboratory criteria. Available evidence suggests that day 7 embryos may, on average, have lower euploidy and implantation rates than day 5 or day 6 embryos, but live births from day 7 embryos do occur.

If you have a day 7 embryo, ask about its full grade, PGT-A result if tested, and the clinic's own experience with day 7 transfers.

How is an embryo selected for transfer to a surrogate?

In gestational surrogacy, embryo selection is generally made by the IVF clinic with the intended parents, within the medical and legal framework of the journey. The goal is to choose an embryo with a reasonable chance of success while protecting the surrogate and pregnancy.

1. Request a complete embryo inventory with developmental day, grade, and PGT-A result when available.
2. Ask how the laboratory ranks day 5 and day 6 blastocysts.
3. Find out whether a medical factor affects the choice beyond the laboratory grade.
4. Confirm the clinic's recommendation on single-embryo transfer. ASRM guidance states that transfer of a euploid embryo should be limited to one, regardless of age.
5. Ask for genetic counseling when results include mosaicism or another complex finding.

Questions to ask your IVF clinic

• How many embryos reached blastocyst on day 5, day 6, and day 7?
• What is the complete grade of each embryo?
• Was PGT-A performed, and what are the limitations of the result?
• How does the laboratory determine the transfer order?
• Does this clinic's own outcome data differ for day 5 and day 6 embryos?
• What is the plan if an embryo does not survive thawing?

The bottom line

Day 5 and day 6 embryos are generally at the same biological stage but reached it at different speeds. Day 5 may carry an average statistical advantage in some settings, but a well-graded or euploid day 6 embryo can still be a strong transfer candidate. The most useful decision combines developmental timing, morphology, genetic results, egg age, and the laboratory's experience.

At Expecting.ai, intended parents can compare options for egg donation and surrogacy and get support planning the journey. You may also want to read our guides to embryo grading and PGT-A in surrogacy.

Professional sources

• ASRM: International Glossary on Infertility and Fertility Care 2025
• ASRM: Blastocyst Culture and Transfer
• ASRM: Guidance on the Number of Embryos to Transfer

This article is for general educational purposes and is not a substitute for personalized medical advice. Embryo selection and transfer planning should be discussed with your fertility physician and embryology laboratory.