After fertilisation, an embryo divides and grows in the laboratory for several days. Blastocyst culture means keeping embryos in culture until about day 5, when they form a blastocyst. This gives the embryologist more information before choosing an embryo to transfer.
What blastocyst culture is
Embryos can be transferred at two main stages: day 2 to 3 (cleavage stage) or day 5 (blastocyst stage). Blastocyst culture keeps embryos in the lab until day 5, sometimes day 6.
By day 5 the embryo is a hollow ball of cells around a fluid-filled cavity. In the UK, about three-quarters of patients now have blastocyst transfers.
| Feature | Day 2 to 3 transfer | Day 5 transfer |
|---|---|---|
| Time to assess | Shorter | Longer, so development is easier to judge |
| Embryos available | More may be available | Some may not survive to day 5 |
| Birth rates (HFEA) | Tend to be lower | Tend to be higher |
| Use in the UK | About one-quarter of patients | About three-quarters of patients |
Culture to day 5 does not create better embryos. It gives embryos time to show how well they develop, which helps the team choose one to transfer.
Who it is for
- People having IVF or ICSI with several good embryos
- People planning single embryo transfer
- People having genetic testing of embryos (PGT), which is usually done at blastocyst stage
If only a few embryos develop, your doctor may discuss an earlier transfer, because embryos may not all make it to day 5.
Blastocyst culture is not right for everyone. Age, the number of embryos and your medical history all matter, so discuss timing with your doctor.
Day by day
- Day 1: fertilised eggThe embryologist checks which eggs have fertilised. The single-cell embryo is called a zygote.
- Days 1 to 3: cell divisionThe embryo divides into 2, 4 and then about 8 cells. Embryologists note the number and evenness of cells.
- Days 3 to 4: morulaThe cells pack tightly together into a ball called a morula.
- Days 5 to 6: blastocystA fluid-filled cavity forms. The inner cell mass will form the baby; the outer trophectoderm will form the placenta.
- Grading and choiceEmbryos are graded on their appearance, such as the number and size of cells and how fast they develop. No method can reliably predict which embryo will become a baby.
- Transfer or freezeOne blastocyst may be transferred, and other suitable blastocysts can be frozen for later.
Cleveland Clinic reports that, on average, about 70% of mature eggs fertilise, and about half of fertilised embryos go on to reach blastocyst over 5 to 6 days. Your own numbers may be higher or lower.
Why day 5, and what it feels like
- Self-selection: embryos that reach blastocyst in the lab are more likely to be viable.
- Better timing: a day 5 embryo is closer to when an embryo would naturally reach the uterus.
- Research: the UK regulator reports higher birth rates with blastocyst transfer, although one 2024 randomised trial found no statistically significant difference.
Blastocyst culture works well with single embryo transfer. The HFEA recommends transferring one embryo for most people, because twins and triplets carry higher risks such as premature birth and low birth weight, and it says single transfers do not reduce success rates.
You will not feel anything during culture because it happens in the lab. The main difference for you is waiting a few extra days for transfer, which is a short, usually painless procedure without anaesthetic.
How to prepare
There is little you need to do differently for blastocyst culture itself. Preparation is mostly about understanding the process and the decisions you may face.
- Ask how your clinic will update you on embryo development each day
- Decide in advance what you want done with extra embryos
- Prepare emotionally for the possibility of fewer embryos by day 5
- Do not compare day 3 cell counts with other people's results
- Do not read an embryo grade as a promise of pregnancy
Clinics may offer extras such as time-lapse imaging, embryo glue or assisted hatching. The HFEA says there is no conclusive evidence that the commonly offered add-ons increase the chance of pregnancy, so ask what evidence supports any extra cost.
Good-quality embryos that are not transferred can be frozen for a later cycle. Some people choose instead to donate them to research or training, so it helps to think about this before treatment.
Risks and limits
| Issue | What sources say |
|---|---|
| Embryos stop developing | About half of fertilised eggs reach blastocyst; some cycles have no embryo to transfer. |
| Fewer to freeze | Fewer embryos may be available for freezing. |
| Twins | Transferring one embryo at a time greatly lowers twin risk; after elective single embryo transfer, twin rates are around 1% to 2%. |
| Cancelled transfer | If no embryo develops to blastocyst, the planned transfer may be cancelled. |
Losing embryos between day 3 and day 5 is common and does not mean you did anything wrong.
How DBN does it
- Embryos are cultured to day 5 (blastocyst) as part of IVF.
- Dr. Tongtis, an OB-GYN since 1983, established the blastocyst culture programme of the Blastocyst Center at Praram 9 Hospital in 1998.
- One embryo is transferred at a time; other embryos can be vitrified and stored at −196 °C.
- IVF & ICSI is about 450,000 THB per cycle including blastocyst culture. First consultation is free; prices are confirmed at consultation.
When to see a doctor
- You are starting IVF and want to understand when your embryos will be transferred
- A previous cycle had no embryos reach day 5
- You are considering genetic testing of embryos
Questions & answers
Is a day 6 blastocyst still usable?
Yes, some embryos reach the blastocyst stage on day 6 rather than day 5. Your embryologist will explain how yours developed.
Why did I have fewer embryos on day 5 than day 3?
Not all embryos can keep developing. Only about half of fertilised eggs reach the blastocyst stage.
Does a top grade guarantee pregnancy?
No. Grading helps choose an embryo, but no method can reliably predict which embryo will become a baby.
Related DBN services
Related articles
Sources
- Decisions to make about your embryos — HFEA
- IVF (In Vitro Fertilization): Procedure & How It Works — Cleveland Clinic
- Blastocyst Development — UNSW Embryology
- Elective single-embryo transfer (eSET) patient education fact sheet — ASRM / ReproductiveFacts.org
- Live Birth after Cleavage-Stage versus Blastocyst-Stage Embryo Transfer: A Randomised Controlled Study (Int J Fertil Steril, 2024) — PubMed Central