However, laboratory advances have meant that we can now culture embryos for five or six days. By this stage, the embryos have numerous cells (approximately 80 – 100) and start to form two distinct layers. The embryos are then known as blastocysts.
Culturing embryos in the laboratory to day five or six gives the embryos more opportunity to prove their developmental potential. Some fertilised eggs may even arrest (stop developing) prior to day 5. This ‘natural selection’ enables the embryologist to more accurately choose the best embryo for transfer which offers the most likely chance of pregnancy.
Blastocyst grading or quality is determined by evaluating the outer ring of cells, known as the trophectoderm or trophoblastic cells, that will eventually form the placenta; the inner cell mass or ICM, which is made up of the stem cells that the baby will develop from.
Scoring of blastocysts is an imperfect science, and some very nice-looking blastocysts do not necessarily produce a pregnancy. However, the basic rule of thumb is that the best embryos make it to the blastocyst stage, and therefore has a greater chance statistically of producing an ongoing pregnancy than a lesser quality one.
During natural conception, eggs and sperm fertilise in the fallopian tube, in which they continue to divide and only reach the uterus on Day 5 post fertilization at this blastocyst stage. Therefore a Day 5 transfer mirrors this physiological timing. Scientific evidence shows that blastocyst transfer success rates are higher than the transfer of day three embryos.
Harley Street Fertility Clinic promotes eSET (Elective Single Embryo Transfer) as recommended by the Human Fertilisation and Embryo Authority (HFEA). This means that there are often additional blastocysts remaining after embryo transfer. If the blastocysts are of good quality, they can be stored for use in a frozen embryo replacement cycle (FET) at a later date. The freezing process is known as vitrification. Currently, scientific reports indicate that the chances of pregnancy using blastocysts that have been vitrified is almost equal to those used in a fresh embryo transfer cycle.