WHAT IS A BLASTOCYST?
In standard IVF, embryos are grown under very strictly controlled conditions in the laboratory for two or three days. They are then transferred into the women’s uterus (womb) at this four to eight cell stage and any additional embryos can be frozen for future use.
However, lab advances have meant now we can keep embryos in the lab for longer periods. Blastocyst culture refers to growing embryos in the laboratory for two more days i.e. day 5 at which point they are referred to as blastocyst embryos. By this time embryos have numerous cells (approximately 80-100) and start to form two distinct layers.
HOW ARE BLASTOCYST MONITORED?
The embryologist looks at the embryos daily to see how many have divided and assess their grading. Blastocysts are graded using a 3 character grading system. Under IVF culture conditions, only 60-70% of human embryos progress to blastocyst stage after 5 days.
Many of the eggs initially fertilized will not progress to four cell stage on day 2 or eight cell stage on day 3 and even less will develop into blastocyst.
The embryos that survive to the blastocyst stage are more likely to be strong, healthy and more likely to result in a healthy pregnancy.
WHY ARE EMBRYOS NOT CULTURED IN LABORATORY BEYOND BLASTOCYST STAGE?
During the course of events in a normal pregnancy around day 5 the embryo starts embedding itself below the surface of the endometrium and keeping the blastocyst beyond this stage will delay the process of embedding in the uterus.
The process of transfer of blastocyst into the uterus is same as an embryo transfer of day 2 or day 3 embryos.
A day three embryo must continue to develop following embryo transfer but a blastocyst will implant much more quickly.
ADVANTAGES OF BLASTOCYST
A lot of embryos are destined to arrest at early stages so extended culture allows the embryologist to identify which embryos have the best potential for implantation by identifying those which form a normal blastocyst.
Transferring embryos at the blastocyst stage also provides a better coordination between the embryo and the uterus by putting the embryo back in the right place at the right time.
Chances of pregnancy are higher at blastocyst stage than for Day 3 embryo.
There is a reduced risk of multiple pregnancy as single or two blastocyst transfer is done according to patient factor.
DISADVANTAGES OF BLASTOCYST
There are certain disadvantages of blastocyst culture also.
The extra culture time required adds to both the financial and emotional cost of treatment.
It is possible that you may not have an embryo transfer at all as sometimes no embryos survive till the blastocyst stage.
It is less likely that there will be spare embryos available for freezing.
There is a slightly increased risk of identical twins reported.
The decision to choose a day 3 or day 5 embryo transfer will depend on the circumstances at the time and previous outcome. Depending on the number and grade of embryos on Day 3 and the patient history, a decision to transfer on Day 3 or proceed to Day 5 is taken. Patients who have fewer eggs retrieved, fewer fertilized or few good embryos on day 3 have no advantage using blastocyst culture.