More and more women seek the help of oocyte freezing as the technology behind oocyte freezing is also getting better and better. Finding the right time of ooctye freezing is one of the crucial factor of higher post-thaw rate of frozen oocytes.

Oocytes are extremely delicate cells. During the process of Vitrofication, to avoid causing damage of the organelles, cell membrane and spindle, the osmotic pressure and temperature are required to be maintain.

Besides, freeing the oocytes at the correct timing not only can avoid harming the spindle but also enhance the survival rate of thawed oocytes. As oocytes freezing becomes a popular option for single unmarried women, embryologists are also eager to know the “tips” regarding the usage of needle and protocol.

In 2019 conference of Taiwanese Society for Reproductive medicine, a reproductive center reported there result of thawing oocytes of 63 clients from 2018 June to 2019 February.

Their research focused on the following factors which might affect the outcome of oocytes thawing:

1. Age

2. Time from trigger of ovulation to oocytes freezing

3. Appearance of the oocytes during the process of temperature balancing.

4. Duration of temperature balancing.

Age

The total number of thawed oocytes from 63 clients is 463. They grouped their clients into 3 groups according their age. The result shows that the difference of post-thaw survival rate of each group has no statistic difference.

Table 1. Post-thaw Survival Rate in Different Group of Age

Age	Number of thawed oocytes	Number of survived oocytes	Survival rate(%)	P value
Total	463	445	96.11	No difference
<38	67	66	98.51	No difference
38~40	169	164	97.04	No difference
41~47	227	215	94.71	No difference

Time from trigger of ovulation to oocytes freezing

To understand if the duration from trigger to oocytes freezing would affect the result of survival rate, they separate the results into 4 groups including <38 hours, 38-39 hours, >39 hours and in vitro maturation (IVM). The result shows that the survival rate of mature oocytes is about 97-100%, and has no difference in different time groups. However, if the oocytes haven’t matured when retrieved, the survival rate with decrease to 71.43%.

Figure 2. Post-thaw Survival Rate in Different Group of duration from trigger to oocytes freezing.

Appearance of the oocytes during the process of temperature balancing

The purpose of temperature balancing is to enforce the exchange between water molecule and cryoprotectant, which can avoid the formation of ice crystal during the process of sudden temperature drop. The ice crystal might harm the oocytes and further affect the survival rate. When the oocytes encounter the cryoprotectant with higher osmoic pressure, the oocytes will shrink and become 3 different formations as shown in Figure 3. The 3 different formations are shrinking to a ball shape(A), shrinking to a wavy shape(B) and irregular shrinking(C). According to the previous research, A is consider the oocytes with better quality.

However, the statistic shows that, there are no significant difference in 3 groups, which also imply that other factors might acquire to take into concern.

Figure 3. Appearance of the Oocyte During the Process of Temperature Balancing

Table 4、Post-that survival rate according to the appearance during the process of temperature balancing.

Appearance of the oocytes	A	B	C
Survival rate(%)	95.00	96.10	96.89

Duration of temperature balancing

Embryologists can justify if the water molecule has exchanged completely or not by observing the appearance of oocytes during the temperature balancing. After the oocytes re-swollen, dehydration and freezing process could be proceeded. The standard of re-swollen time is around 12-15 minutes. Although the time of re-swollen might be shorter in realistic, the time of re-swollen is not affective to the post-survival rate also.

Figure 3. Survival rate of different duration of temperature balancing.

Coincidentally, Stork Fertility Center also published a similar research in 2019 ASPIRE. However, our result is slightly different from the previous research. Our subject is to determine whether the time of oocytes freezing is relevant to the pregnancy rate when using GnRH as the trigger. We back tracked 396 cycles and 7224 oocytes from our oocyte recipient program in 2017 Oct to 2018 Oct. 3313 out of 7224 oocytes were frozen 38 hours after trigger. 3911 oocytes were frozen over 40 hours.

As shown below, the outcome of using oocytes frozen 38 hours after trigger are slightly better. The benefits are shown especially in survival rate and good blastocyst rate with significant difference.

We also further compare the conception outcome by using two group of oocytes. The result shows that the pregnancy rate and the implantation rate are not relevant to the time from trigger to oocyte freezing, which indicates that there’s no need to underestimate the potential of the blastocyst no mater it’s from which kinds of oocytes.

Oocyte freezing service is one of the greatest promise we made for our clients. Therefore, we always discretely preserve the precious oocytes of our clients. For higher age, diminished ovarian reserve and lower oocytes mature rate clients, we will check the spindle additionally before freezing the oocytes.