Blastocysts: 10 things IVF patients should know

26 Apr Blastocysts: 10 things IVF patients should know

Blastocysts have an image problem – and not just because they sound like a bad day at NASA. Should IVF patients transfer one or more? Is it a good idea to culture embryos for five days, not three? Are multiple births more or less likely? Here’s our round-up of the top 10 things you need to know about blastocysts – based on the most trusted research.

1. What is a blastocyst?

A blastocyst is a human embryo that’s five or six days old. Ten years ago, day-three embryos were routinely transferred in IVF cycles. Most clinics now believe that transferring better-developed embryos – i.e. those that have reached the blastocyst stage – makes an ongoing pregnancy more likely.

2. What does a blastocyst look like?

Under the microscope, a blastocyst is very different to a day-three embryo. There’s a fluid-filled cavity in the middle called the blastocoel. Another area, housing a dense collection of cells, is called the inner cell mass and will become the fetus. A third assortment of cells, near the central cavity, will form the placenta.

3.  Is blastocyst transfer preferable to a day-three transfer?

The consensus is that an embryo that survives for five days is more viable for implantation and an ongoing successful pregnancy. Of course, many embryos don’t make it to day five. Where there’s a serious sperm-quality issue, for example, embryos will often stop developing by day three.

We also know that a better diet can help embryos get to day five – and numerous other studies point to the benefits of blastocyst formation and culturing. Blastocyst transfers can lead to better outcomes than earlier transfers.

4. Any other advantages in transferring blastocysts?

Blastocysts, on a purely biological level, can make things more straightforward for implantation. In natural conception, an egg will travel down a fallopian tube and be fertilised – hopefully. It may also connect to your endometrium at the right time – if you’re lucky. Placing a blastocyst directly into the womb, optimally timed, in a medicated cycle, between day 19 and 21, is an easier prospect for the hormonally-primed uterus.

5. Why don’t all IVF patients use blastocysts?   

You may not get to day five. Depending on factors like your medical history, age, treatment choice, egg and sperm quality, your clinic may feel it’s safer to transfer a younger embryo. Or it may be the only option. Better to put in a tiddly one than none at all.

It’s also worth noting that good-quality blastocysts are more likely in donor-egg and donor-embryo cycles. Looking back at the donor patients we’ve helped, day-five blastocyst transfers have taken place on almost every occasion. IVF cycles can lead to earlier transfers, since gamete quality is less guaranteed. More can go wrong. But even then, day-five transfers in own-egg cycles happen a lot.

6. So do blastocysts lead to more IVF pregnancies?

Generally, yes. If you’re under 35, established research says that live birth rates after blastocyst transfer are higher than after cleavage-stage (earlier) transfers. Clinic data in the UK and across Europe shows that blastocyst transfers significantly boost clinical-pregnancy and live-birth rates. Typically we see pregnancy rates around 20 percentage points higher after day-five transfers. This applies across all age groups and all treatments. Cultivating a good number of top-quality blastocysts has also been shown to be a good predictor of future success.

7. Are there any drawbacks with blastocyst transfers?

Some patients worry that they’ll have fewer or no surplus embryos to freeze. This is a false argument. It’s better for get to day five and know your embryo quality. Yes, some embryos will be left behind. But you need to know that. With any luck, you’ll transfer the best one and have one or two good ones to freeze.

Freezing of embryos for a second attempt is often overlooked by IVF patients. The fact is that frozen embryo transfers, particularly those using blastocysts, are almost as successful as fresh ones. So here’s a tactic. If you have, say, two blastocysts on day five, freezing one and transferring the other may be sensible. In our case, each time we transferred two blastocysts, only one implanted. But we were in our 40s, an age range where implantation rates are  a bit lower. Read about our success story here.

8. Do I transfer one blastocyst or two?

It’s firmly established that single embryo transfer is the way to go. Advances in extended cultivation technology and cryopreservation mean better quality embryos at the blastocysts stage and after freezing. The days of multiple embryo transfers are over, due to the well-known risks associated with multiple pregnancies. Read our blog post on this.

9. Twins after a single blastocyst transfer… what’s going on? 

Blastocysts can split more easily than younger embryos, especially good-quality ones. So even a single blastocyst transfer can lead to twins (though it’s not common). The most dramatic scenario is if, after a double transfer, both blastocysts split and progress to live birth. Quadruplets, anyone? (This is very rare.)

10. So are blastocysts a good thing or not?

Yes, blastocyst transfer is the best option for most couples. In the distant past, some clinics would transfer up to four day-three embryos and hoped for the best. Blastocysts allow for more informed choices and better control over outcomes. While a 2019 study suggested blastocyst transfers increased the risk of premature birth and heavier babies, that risk was small. The routine approach to day-five transfers will continue.

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