世界上第一個通過體外受精出生的嬰兒今年已經42歲了。但42年後的今天，體外受精還是一個相對較新的領域，人們一直在爭論如何為那些將希望（通常是個人儲蓄）投入生育治療的家庭取得最佳效果。
體外受精專家對將新鮮胚胎或冷凍胚胎移植到患者子宮是否為健康嬰兒提供了最佳機會存在分歧。根據2018年8月20日發表在《生育與不孕》雜誌上的一項對近8.3萬名體外受精患者的研究，其結果表明目前還沒有一種萬能的解決方案。到底哪種方法更好取決於患者產生的卵子數量。
對於新鮮胚胎的移植，患者通常需要服用激素數周來刺激產卵。醫生在相同的過程中取回卵子，在體外使卵子受精並將一個或多個胚胎移植在母親體內。但現在許多診所普遍建議冷凍所有胚胎，並等待幾周患者進入新的月經周期後移植到母親體內。
杜克大學生殖內分泌學和不孕不育專家蘇海爾穆阿舍（Suheil Muasher）說：「過去五年，有些診所提倡冷凍所有的卵子，他們認為激素刺激產卵的過程會降低子宮內膜的受孕率」。他表示，有些研究還表明移植冷凍胚胎不太可能會導致早產和低出生體重兒。
但根據杜克大學的研究，冷凍胚胎移植可能僅對激素刺激後產生十五個以上卵子的女性有利。這些接受冷凍胚胎移植的所謂「高應答者」的出生率（52％）略高於接受新鮮胚胎移植的那些人（48％）。
然而，與接受冷凍胚胎的婦女相比，低應答和中等應答的婦女即產生十五個以下卵子的婦女的新鮮胚胎移植的妊娠率和出生率更好。杜克大學生殖內分泌學和生育學研究員、該項研究的主要作者凱利·阿查裡亞（Kelly Acharya）說，向這些人提供新鮮胚胎移植可能還有其他優勢。
她說：「從患者的角度來看，如果他們能夠繼續進行一次新鮮胚胎的移植，就可以省去額外的等待時間和新一輪的激素治療。」 「冷凍胚胎可能導致需要再等一兩個月，並且該程序是否會成功還是未知數，這可能會使患者情緒低落。從這些數據來看，冷凍胚胎似乎沒有任何好處。」
該研究的數據來自2014年-2015年間輔助生殖技術學會登記冊中的首次體外受精患者。低應答者產生1-5個卵子，中應答者產生6-14個卵子，高應答者產生十五個以上的卵子。
Muasher說，以往的研究主張「全部冷凍」的方法有其局限性，因為納入研究的患者人數相對較少。他認為，大樣本的杜克研究可以更準確地代表美國接受體外受精治療的患者。但杜克研究的局限性在於作者無法評估患者選擇冷凍胚胎移植而非新鮮胚胎移植的原因。
Acharya說：「到底哪種方式更好還需要進一步的研究，但患者可能會看到很多信息，這些信息表明冷凍胚胎移植始終是最好的方式，而我們看到的情況可能並非如此。如果他們是低反應或中度反應者且正在進行新鮮胚胎移植，希望這篇文章能讓他們放心。」
特别致謝 編譯：橙敏，來源：sciencedaily
For women undergoing IVF, is fresh or frozen embryo transfer best?
Some women benefit from a fresh embryo transfer while others benefit from delay
The world's first baby born via in-vitro fertilization turned 40 years old this summer. Still, after four decades, IVF is a relatively new field with ongoing debate on how to get the best results for families who have placed their hopes -- and often their personal savings -- into fertility treatment.
IVF experts disagree about whether transferring a fresh or frozen embryo to a patient's womb offers the best opportunity for healthy babies. According to a study of almost 83,000 IVF patients published August 20 in the journal Fertility and Sterility, there is no one-size-fits-all solution. The best technique may vary, depending on how many eggs the patient produces.
For a fresh transfer, patients take hormones for several weeks to stimulate egg production. Doctors retrieve the eggs, fertilize them and place one or more embryos in the mother during the same procedure. But many clinics now universally recommend freezing all embryos and waiting a few weeks for the patient to enter a new menstrual cycle.
"In the past five years, some clinics have advocated for freezing everything with the belief that the process that stimulated the eggs makes the lining of the uterus less welcoming for pregnancy," said Suheil Muasher, M.D., a reproductive endocrinology and infertility specialist at Duke and senior author of the paper. Some research has also indicated frozen embryo transfers are less likely to result in preterm labor and underweight babies, he said.
IVFStockPhoto_web.jpgBut according to the Duke study, waiting may be advantageous only for women who produce 15 or more eggs after hormone stimulation. Birth rates for these so-called 'high responders' who received frozen embryos were slightly higher (52 percent) than those who received fresh transfers (48 percent).
However, in low and intermediate responders -- women who produced 14 eggs or fewer -- fresh transfers led to better pregnancy and birth rates compared to those who received frozen embryos.
There may be other advantages to offering fresh transfers to these groups, said Kelly Acharya, M.D., a fellow in reproductive endocrinology and fertility at Duke and the study's lead author.
"From the patient perspective, if they can go ahead and do a fresh transfer it saves them the additional waiting and having to take another cycle of hormones," she said. "Freezing also can lead to another one or two months of waiting and not knowing whether the procedure will be successful, which can be emotionally draining for patients. From these data, it doesn't look like there is any benefit to waiting."
The data in the study came from first-time IVF patients included in a registry of the Society for Assisted Reproductive Technology between 2014 and 2015. Low responders produced one to five eggs, intermediate responders produced six to 14 eggs, and high responders produced 15 or more.
Previous research advocating a "freeze-all" approach has been limited, in that the numbers of patients included in the studies have been relatively small, Muasher said. He suggests that the large Duke study is a more accurate representation of patients undergoing IVF treatment in the U.S. However, one limitation of the Duke study was that authors were unable to assess the reasons patients opted for a frozen embryo transfer rather than a fresh one.
Further research is needed, Acharya said, but "the bottom line for patients is that they're likely seeing a lot of information out there saying frozen transfer is best all of the time, and we are seeing that may not be the case. Hopefully, this could put some patients' minds at ease if they are a low or intermediate responder and doing a fresh transfer."