The practice of freezing embryos as part of in vitro fertilization (IVF) was thrown into chaos in Alabama this year, when the state supreme court ruled that such embryos should be considered children, exposing clinics to wrongful death claims in the event they are destroyed in the thawing process.
In 2021, more than 80 percent of US IVF procedures involved the transfer of frozen embryos, according to a recent report by the US Department of Health and Human Services.
HOW IS EMBRYO FREEZING USED IN IVF?
Photo: Reuters 照片：路透
IVF uses high doses of hormones to stimulate ovaries to produce as many eggs as possible. Once the eggs are extracted, the most mature are selected for attempts at fertilization with sperm.
Over the next 5 to 6 days, healthy fertilized eggs grow into blastocysts — the earliest stage of embryo — containing roughly 100-200 cells. Blastocysts can be transferred into the uterus or frozen to be thawed for later use.
After transfer into the uterus, if all goes well, the blastocyst implants itself in the wall of the womb and continues to grow.
Photo: AP 照片：美聯社
Typically, if 20 eggs are retrieved after ovarian stimulation, about 16 will be mature, about 12 of those will fertilize when combined with sperm, and perhaps six will grow into healthy blastocysts with good potential for implanting and resulting in a successful pregnancy, said Dr. Zev Williams, Chief of the division of reproductive endocrinology & infertility at Columbia University Fertility Center in New York.
WHAT ARE SOME BENEFITS OF USING FROZEN EMBRYOS IN IVF?
For some patients, freezing embryos and then waiting several weeks before a transfer into the uterus is more likely to lead to successful implantation, depending on their age, underlying health or hormone levels.
Photo: AP 照片：美聯社
The pause allows the body’s hormone levels to normalize following ovarian stimulation. It also reduces the risk of ovarian hyperstimulation syndrome (OHSS), a potentially life-threatening effect of intense hormone use.
More broadly, freezing embryos often means that only one course of painful, expensive ovarian stimulation and egg retrieval is needed. If an embryo transfer fails, additional embryos can be thawed and used.
Pregnancies with twins or triplets were more common with IVF before embryo freezing, as doctors would transfer more than one embryo at a time to increase the chances of a successful pregnancy.
Photo: AP 照片：美聯社
Freezing of embryos also allows patients to preserve fertility in advance of chemotherapy or other treatments that can damage the reproductive organs.
Genetic screening of embryos before a transfer is only possible with freezing because it can take several weeks to get results. It is often employed when there is a history of recurrent miscarriages, previous IVF failures and maternal age over 35 or a family history of genetic diseases.
WHAT IF FREEZING EMBRYOS FOR IVF BECAME UNAVAILABLE?
The loss of the option to freeze embryos for later usage “would represent a significant setback for the field,” said Dr. Steven Spandorfer of Weill Cornell Medicine in New York City, president of the Society for Assisted Reproductive Technology.
A single pregnancy is the best way to promote the birth of a healthy child after IVF, so clinics are unlikely to return to the practice of transferring multiple embryos into the womb, Spandorfer said.
Clinics could freeze eggs instead of embryos, but that approach has many limitations that would lower the overall success of IVF.
The viability of those eggs would not be clear until they are individually thawed and IVF is attempted, creating the potential for delayed embryo transfers and the need for additional hormone use and retrievals.
(Reuters)
冷凍胚胎用做體外人工受孕（IVF），此做法今年在美國阿拉巴馬州陷入混亂，因為該州最高法院裁定此類胚胎應被視為兒童，若這些胚胎在解凍時毀壞，診所將面臨不法致死之索賠。
根據美國衛生及公共服務部最近的報告，2021年美國80%以上的IVF程序涉及冷凍胚胎植入。
胚胎冷凍如何用於體外人工受孕？
體外人工受孕是用高劑量的荷爾蒙來刺激卵巢產生盡可能多的卵子。取出卵子後，挑選最成熟的來與精子受精。
接下來的5至6天，健康的受精卵會發育成囊胚（胚胎的最早階段），含有約100至200個細胞。囊胚可移植到子宮，或加以冷凍，供未來解凍後使用。
植入子宮後，若一切順利，囊胚會自行在子宮壁著床，並繼續成長。
一般而言，卵巢經刺激後若取出20個卵子，大約16個會成熟，其中約12個會與精子結合受精，6個可能會發育成健康的囊胚，具有良好的植入潛力並促成懷孕，紐約哥倫比亞大學生殖中心生殖內分泌與不孕症科主任澤夫‧威廉斯博士說。
體外人工受孕使用冷凍胚胎有何好處？
對一些患者來說，將胚胎冷凍，等待數週再植入子宮更有可能讓胚胎成功著床，這取決於其年齡、基本健康狀況或荷爾蒙濃度。
這種暫停可讓身體的荷爾蒙濃度在卵巢刺激後恢復正常。它還可降低「卵巢過度刺激症候群」（OHSS）的風險，這是因大量使用荷爾蒙而引起的併發症，可能危及生命。
更廣地來說，將胚胎冷凍往往表示刺激卵巢、取出卵子這痛苦且昂貴的過程，只需經歷一次。若胚胎植入失敗，可將額外的胚胎解凍使用。
雙胞胎或三胞胎在胚胎冷凍前進行的體外人工受孕更常見，因為醫師會一次植入多個胚胎以增加懷孕成功的機會。
將胚胎冷凍還可讓患者在化療或其他可能損害生殖器官的治療進行前，將生育能力保留下來。
胚胎植入前對其進行基因篩檢只能透過冷凍進行，因為可能需要數週才能得知結果。若曾反覆流產、體外人工受孕失敗且產婦年齡超過35歲或有遺傳疾病家族史，通常會採用此方法。
若胚胎無法冷凍，IVF將如何進行？
人工生殖技術協會主席、紐約市威爾康奈爾醫學院的史蒂文‧史班多弗博士說，失去冷凍胚胎供以後使用這個選項「將重挫該領域」。
史班多弗說，單胎懷孕是體外人工受孕生出健康孩子的最佳方式，因此診所不太可能恢復將多個胚胎植入子宮的做法。
診所可以冷凍卵子而不是胚胎，但這種方法有許多限制，會降低體外人工受孕的整體成功率。
這些卵子的活力只有在被單獨解凍並嘗試體外人工受孕後才能清楚，這可能會延遲胚胎植入的時程，增加賀爾蒙的使用及取卵次數。
（台北時報林俐凱編譯）
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