Author Name: Dr. Pooja Singh || Mentor Name: Dr. Rinoy Sreedharan on April 27, 2020
IVF or “In Vitro Fertilization” is an Assisted Reproductive Technique in which mature eggs are retrieved from the ovaries of a female (after a course of medication in the form of tablets or hormonal injections) followed by fertilisation of these eggs in a laboratory with sperms. These fertilised eggs are then cultured in the laboratory for three to five days to form embryos which are then transferred to the uterus for further development.
The ultimate desired outcome of any IVF technique is to get a high live birth rate. There are many factors which determine the success of IVF, and among which the number of eggs retrieved following ovarian stimulation is considered to be one of the most important.
Ovarian stimulation is an integral part of IVF procedures. It is a process in which the follicles in a woman’s ovaries are stimulated to increase their size by giving daily gonadotrophin injections. When the size of majority of follicles reach the desired size of around 18-20 mm, a trigger injection is given for maturation of follicles, which is followed by the egg pick up/ retrieval procedure. The goal is to collect as many mature oocytes as possible. It is important to understand that during the process of stimulation there are no new eggs formed. Instead, the growth of existing eggs is promoted by giving adequate higher doses of hormonal injections.
Earlier, the aim of ovarian stimulation in IVF cycles was to try to get as many mature oocytes as possible, because not all oocytes retrieved will form embryos after fertilisation. Similarly, not all embryos will be of good quality in spite of best laboratory conditions. So it was believed that higher the number of eggs retrieved, greater are the chances of achieving a pregnancy.
However, maximising ovarian stimulation with the desire to yield maximum oocytes has its own adverse consequences. Ovarian hyperstimulation syndrome (OHSS) is the most important of them.
With the advent of newer stimulation protocols and shift towards the use of safer injections for trigger, the chances of having OHSS is greatly reduced .
With the advent of freezing techniques , the desire to retrieve more oocytes in a single stimulation cycle increased. In many centers, embryos are cultured till blastocyst stage and then one or two of them are transferred and the remaining embryos are frozen for future purposes. So, more the number of oocytes, greater is the number of embryos formed and higher are chances of using them in consequent cycles. Hence, it ultimately increases the cumulative live birth rate of IVF.
Age of the female is also a very important parameter which decides the mimimum number of eggs are needed to get a healthy pregnancy. Various studies have shown that to produce 1 euploid (genetically normal) embryo the number of eggs needed increases exponentially with age. For example at the age of 34yrs, 5 oocytes will be needed to make 1 euploid embryo. However at the age of 38 yrs, 10 oocytes will be needed for the same. Similarly to produce 2 euploid embryos 14 and 24 oocytes will be needed at 34 and 38 yrs of age respectively. So we can say that the minimum number of eggs needed for a live birth increases with female age.
There is a second school of thought which suggests that the live birth rate increases with number of eggs retrieved upto around 15 eggs, and then the rate decreases if more than 15 eggs are retrieved in a single ovarian stimulation cycle. Hence they opine that there is a non linear increasing relationship between number of eggs and live birth. Especially in case of fresh transfers, because higher number of eggs leads to higher estrogen value and hence low implantation rate.
Newer studies have shown that the quality of eggs and age of the female are more important determining factors in IVF success than the number of eggs in total.
So we can conclude that undoubtedly the number of eggs retrieved are important for success of IVF but in modern era the focus of IVF centers is shifting from quantity of oocytes to quality of oocytes and also to the female age, with the ultimate desire to obtain a higher cumulative live birth rate.
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