HOW MANY EMBRYOS TO TRANSFER WITH ASSISTED REPRODUCTIVE TECHNOLOGY?

Assisted reproductive technology (ART) procedures may require the transfer of embryos to a woman’s uterus to start a pregnancy. Embryo transfer from the laboratory to a woman’s womb is an important step, and choosing the optimal number of embryos will help prevent medical complications.

The transfer of many embryos can result in multiple pregnancies, which is considered high risk. Multiple pregnancies are associated with uncomfortable complications for the mother, which includes gestational diabetes, high blood pressure and early labor. In addition, multiple pregnancies may cause premature birth, leading to respiratory complications, feeding problems and increased chances of infection in the fetus.  Therefore, the ideal outcome of ART procedures is pregnancy with a single fetus.

What the Authorities Say? Fertility clinics must follow the guidelines of the Society of Assisted Reproduction (SART) and the American Society of Reproductive Medicine (ASRM) when deciding the optimal number of embryos to transfer.

Generally, the number of embryo that can be transferred increases with an increase in the woman’s age, to compensate for age-related decline in fertility. Both embryos (between days 2 – 3 post-fertilization) and blastocysts (between days 5 – 6 post-fertilization) may be considered for transfer into a woman’s uterus. Blastocyst transfers are known to be more efficient than embryo transfers, and therefore, a fewer number of blastocysts are sufficient for ART purposes.

According to the accepted guidelines, women below the age of 35 require 1 – 2 embryos, or 1 blastocyst per transfer. On the other hand, woman above 40 years of age can have as many as 5 embryos, or 3 blastocysts per transfer.

It is, however, recommended by the authorities that fertility practices may use more or less than the recommended limit of embryos per transfer, in order to address a patient’s unique requirements. Apart from a woman’s age, the quality of the embryo / blastocyst also determine the overall success of embryo transfer.

Embryo / Blastocyst Quality: To understand the difference between an embryo and a blastocyst, one has to consider the gradual development of a fertilized egg, between 5 – 6 days post-fertilization. The fertilized egg, also known as zygote, undergoes cellular division to become a two-cell embryo, and later a four-cell embryo, and so on.

As the embryo continues to grow and divide, the cells start to become compact and a Morula is formed. Subsequently, further development leads to the formation of a blastocoele cavity—this is the early blastocyst stage. A Full blastocyst is marked by the development of an inner cell mass.  Later on, an Expanded Blastocyst is formed, which represents a highly developed embryo. It is known that 20 – 50 % of all embryos successfully develop into the blastocyst stage.

A high quality embryo / blastocyst, as determined by an embryologist through visual observation, has the best chance of generating a favorable outcome. A Gardner system of grading is followed by many clinics to select the healthiest embryos. Based on this system, the number of cells and the amount of fragments present within the embryo are attributes that determine overall embryo quality.