In vitro fertilisation usually involves the transfer of embryos two to three days after they have been successfully fertilised. Blastocyst embryo transfer is a variation of IVF in which an embryo is allowed to develop for five to six days, by which time it has become a ball of about 120 cells known as a blastocyst.
Having blastocyst embryo transfer as part of your IVF cycle makes very little difference to your overall treatment. You just have to wait 5–6 days instead of 2–3 days after your eggs have been fertilised before an embryo can be transferred into your uterus.
There is some extra stress involved as not all fertilised eggs develop into blastocysts when they are fertilised and then cultured outside the body. In the usual culture fluid used in IVF, only about 20 % of embryos turn into blastocysts within 6 days. This can be increased to about 60 % by using different types of culture fluid as the embryo starts to develop. However, it is not unusual for your embryos not to develop into blastocysts.
Fortunately, they can still be implanted, and you may still become pregnant if your embryos appear to be healthy and good quality.
It is useful to compare the success rates for blastocyst embryo transfer with those achieved when fresh (not frozen and thawed) 3-day embryos are transferred in a more traditional IVF cycle.
Data published by the Human Embryo and Fertilisation Authority (HFEA) showed that in 2009, 32.3 % of women under 35 treated with routine IVF became pregnant and had a live birth. The proportion of women younger than 35 who got pregnant after blastocyst embryo transfer was 47.9 %, which is a difference of 15.6 %. Similar increases were seen in older women (see the table below).
|Age of women||% of IVF cycles that achieved a successful pregnancy||% difference in success rate|
|2-3 day embryos||blastocyst embryo transfer|
|35-37||27.2 %||44.6 %||17.4 %|
|38-39||19.2 %||34.1 %||14.9 %|
|40-42||12.7 %||26.8 %||14.1 %|
|45 or older||1.5 %||*||-|
*Numbers of women too small to produce data
The number of women who have blastocyst embryo transfer is quite small compared to the number of IVF cycles done with 2–3 day embryo transfer over the years, but it does appear to increase the chances of pregnancy.
As blastocyst embryo transfer increases your chance of pregnancy significantly, most infertility clinics and specialists recommend transferring only one blastocyst to avoid a multiple pregnancy. Spare blastocyst embryos that are good quality can be frozen and then thawed for use in a subsequent IVF cycle.
Any woman undergoing IVF treatment may be offered blastocyst embryo transfer; it will be suggested by your medical team after they have assessed the number and quality of your embryos at day 3. If there are only two high quality embryos, you are most likely to have those embryos transferred.
If you have several high quality embryos that are all growing really well, it may be best to wait until day 5 or 6 and have the best quality embryo or blastocyst transferred then. This will be done if it is likely to give you a greater chance of achieving pregnancy.
In general, blastocyst embryo transfer is recommended, if it is possible, in younger women who are judged to have a good chance of success with IVF, and also in women who have had a couple of failed IVF cycles, despite having good quality embryos transferred at day 2–3.
ICSI, intracytoplasmic sperm injection is a technique used as part of IVF if the male partner does not produce either many sperm, or enough good quality sperm. Instead of mixing the sperm and eggs together in vitro, the DNA from a single sperm is injected into each egg to bring about fertilisation.
The embryos that result are cultured as they are in an ordinary IVF cycle, and the same decision to wait for blastocysts to form can be made at day 2–3. If these develop well by day 5–6, blastocyst embryo transfer can then take place.