Frozen embryo transfer is a controversial topic in assisted reproductive technology. It is intuitive to think that fresh embryos should be better than ones that have to experience freezing and thawing. But there is also unneglectable scientific evidence supporting frozen embryo transfer.
Some doctors believe that the process of stimulating the ovaries with hormones for egg retrieval makes the uterine lining less suitable for welcoming a pregnancy straight away. Thus, having the embryos frozen and transferred at a later cycle is more advantageous.
However, others argue that having patients wait for another one or two months before the frozen embryo is transferred can also be emotionally draining, doing more harm than good. Plus, wouldn’t freezing the embryo damage its quality?
What is a frozen embryo transfer?
A frozen embryo transfer is when eggs collected after ovarian stimulation are frozen by vitrification – a rapid freezing technique. The embryos are then stored in liquid nitrogen at -196 ° C until you decide to use, donate or discard them. Thawing the embryos involves moving the embryos from liquid nitrogen and replacing the cryoprotectant with water. The process is simple and takes about 15 minutes.
Who may need to freeze embryos?
Women undergoing fertility treatment: The reality is that IVF is physically and emotionally demanding. Going through egg retrieval every cycle can be very challenging. Thus, a common practice is to retrieve as many good-quality eggs as possible during the first IVF cycle. Having some spare eggs to fertilize and then freezing the embryos makes it easy to transfer in the future if the first cycle didn’t work out.
Women who need to preserve fertility: Your fertility is at risk if you have a serious illness such as cancer or are not in a position to have a baby at the moment. Freezing embryos could preserve your fertility before it declines.
Frozen vs Fresh embryos - Success rates
A number of studies in the past decade have compared the success rates of frozen embryo transfers versus fresh embryo transfers. However, the answer is not a straightforward one.
A study in 2017 examined 1892 women and found no clear evidence of a difference in the live birth rate between the two transfer methods. If the cumulative live birth rate is 58% following a fresh IVF embryo transfer, the rate following a frozen transfer strategy would be between 56% and 65% (1).
But in a larger study published on the Journal Fertility and Sterility in 2018, 83,000 IVF patients were involved and the conclusion was: There is no one-size-fits-all solution. The better choice may vary depending on how many eggs the patient produces.
The study subdivided the participants into cohorts based on the number of oocytes retrieved: 1-5 (low), 6-14 (intermediate), and 15+ (high responders). High responders were found to have a higher clinical pregnancy rate and live-birth rate with frozen embryo transfers. However, the opposite was true for intermediate and low responders (2).
These trends are corroborated by two more studies that concluded that: The frozen embryo transfer is most beneficial when the number of eggs retrieved is greater than 15, or most significantly, 20. (3,4).
Thus, it seems that for women who experience ovarian hyperstimulation, having the embryo frozen and transferred at a later time would be more beneficial. But otherwise, fresh embryo transfers may be better.
Frozen vs Fresh embryos – Health implications
Apart from success rates, the health implications for both mothers and babies are important.
The most comprehensive systematic review and meta-analysis on this topic to date were conducted in 2018 (5). It investigated 26 studies and found that babies conceived from frozen embryo transfers were at a lower risk of preterm delivery, low birth weight and small for gestational age. However, these babies are at an elevated risk of large for gestational age of the pregnancy and high birth weight. In terms of the risk of congenital animalities and perinatal mortality, the two transfer methods show similar results.
Among infertile women with polycystic ovary syndrome (PCOS), frozen embryo transfer has been shown to have a higher birth rate and a lower risk of ovarian hyperstimulation syndrome compared to fresh embryo transfer. However, the risk of preeclampsia is higher with frozen embryo transfer (6).
Thus, there is again no black-and-white answer as to which transfer method is superior. Both positive and negative health implications are associated with frozen embryo transfer.
So which one to choose, frozen or fresh embryo transfer?
As we can see from the studies discussed above, there are both pros and cons for each transfer method. Let’s summarise the findings so that you can make a personal choice based on your conditions.
Frozen embryo transfer has better health outcomes for women who have ovarian hyperstimulation syndrome (typically >15 eggs retrieved) or PCOS.
Fresh embryo transfer has better success rates if the number of retrieved eggs is lower than 15.
Frozen embryo transfer is associated with lower risks of preterm delivery and low birth weight, but higher risk for high birth weight.
Ovarian stimulation may affect the uterine lining during an IVF cycle and make immediate fresh embryo transfer and pregnancy unfavorable.
Frozen embryo transfer makes it unnecessary to through ovarian stimulation and egg retrieval again if the first IVF cycle is unsuccessful.
Frozen embryo transfer is more cost-effective if you require multiple IVF cycles.
Frozen embryo transfer is suitable if your goal is fertility preservation.
Frozen embryo transfer is generally the choice if you require embryo donation.
Fresh embryo transfer is better if you wish to finish a full IVF cycle quickly.
References
1. K.M. Wong, M. van Wely, F. Mol, S. Repping, S. Mastenbroek Fresh versus frozen embryo transfers in assisted reproduction Cochrane Database Syst Rev, 3 (2017), p. CD011184
2. Xu, Bing, He, Ya-Qiong, Wang, Yuan, Lu, Yao, Hong, Yan, Wang, Yao, & Sun, Yun. (2017). Frozen embryo transfer or fresh embryo transfer: Clinical outcomes depend on the number of oocytes retrieved. European Journal of Obstetrics and Gynecology, 215, 50-54.
3. Chen, Zi-Jiang, Shi, Yuhua, Sun, Yun, Zhang, Bo, Liang, Xiaoyan, Cao, Yunxia, . . . Legro, Richard S. (2016). Fresh versus Frozen Embryos for Infertility in the Polycystic Ovary Syndrome. The New England Journal of Medicine, 375(6), 523-533.