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In previous years of in-vitro fertilization research, it was thought that fresh embryo transfers were superior to frozen embryo transfers and that frozen embryo transfers should be avoided and only used when there was no other option. New research in recent years however has changed the data on fresh and frozen embryo transfer cycles and the effectiveness of each in terms of pregnancy and live birth rates. This has in turn impacted many providers’ decisions to perform a “freeze-all” delayed embryo transfer versus a fresh embryo transfer.

Many people may become confused about the terminology regarding frozen embryo transfers. A “freeze-all” approach is where all embryos from the resulting IVF cycle are cryopreserved (frozen) and a fresh embryo transfer is not attempted until a later date. This is different from freezing extra or leftover embryos which are the result following a fresh IVF transfer.

The three main things that affect the pregnancy and live birth rate following IVF treatment are the receptivity of the endometrial lining(uterus lining), the quality of the embryos, and the ease of the embryo transfer. If all three of these factors are optimal, the pregnancy and live birth rate is going to be highest possible.

In many situations, some specialists feel that fresh embryo transfers are still preferred and should be attempted if at all possible. However there are several situations in which a frozen embryo transfer may be more ideal and result in a higher success rate for the recipient. Some specialists may prefer a freeze-all approach to all patients undergoing IVF. Below are some examples which may impact the clinician’s decision to freeze all or pursue a fresh embryo transfer.

Ovarian hyperstimulation syndrome risk

If the patient has an excessive amount of follicles and elevated estradiol level produced as a result of the IVF treatment, this puts her at risk for ovarian hyper-stimulation syndrome or OHSS. Luckily through advances in technology, the risks of a serious medical consequence resulting from hyper-stimulation is low however research shows that in cases where the estradiol level and follicle count is increased, a  frozen embryo transfer is the more preferred choice. This may mean freezing all embryos and not attempting a fresh embryo transfer until after the woman’s hormone levels have returned to normal following IVF stimulation.

Elevated progesterone Level

Increased exposure to progesterone during IVF stimulation may make the endometrium(uterus lining) less receptive due to changes that occur within the uterine lining. Typically if the progesterone level is high on the day of the HCG trigger, it is preferred to freeze all embryos at the retrieval and perform a delayed embryo transfer. Research has shown that in cases where the progesterone level is high before the HCG trigger, it has resulted in a lowered pregnancy rate.

Less than optimal embryos

Women who have produced embryos which are less than optimal in appearance may benefit from a frozen and delayed embryo transfer. Less than optimal embryos may also be “slow growing” taking longer to divide than a more optional embryo. The theory behind this method is that the endometrium may be more receptive to accept the embryos in a delayed frozen cycle versus a fresh transfer.

Recipient age

In cases where the recipient is under age 35, it is preferable to perform a fresh embryo transfer as typically the egg quality in women under 35 is better as compared to someone of more advanced age. If the woman is over age 35, research shows that it is preferable to freeze all the embryos and perform a frozen embryo transfer at a delayed date.

This approach may come with risks to the recipient as women over age 35 who undergo IVF, may only have one or a small number of embryos as a result. With advances in assisted reproductive technology and lab guidelines, the risk of damaging or losing an embryo during the freezing and thawing process is low. However, this is still a risk to consider and of if the woman only has one embryo available as a result of the IVF treatment, she and the physician may agree that a fresh embryo transfer poses less risk.

Egg donor population

In cases where the IVF recipient has used an egg donor in the process, research shows that a fresh embryo transfer is preferred over a frozen approach. This is mainly due to the young age of the egg donor and the receptivity of the recipient’s uterus.

Mixed protocol IVF

In some IVF centers, they use a mixed protocol of both follicle stimulating hormone (FSH) in combination with oral clomid. In situations where a mixed protocol is used, research favors freezing all embryos and delaying the embryo transfer versus performing a fresh embryo transfer. A mixed protocol can cause an anti-estrogen effect,(because clomid can affect the uterine lining) and reduce uterine receptivity therefore it is better to freeze and delay when using a mixed protocol.

Endometrial insufficiency

In cases where the endometrium has not reached an optimal thickness or contains mucus or other fluid on the day of the scheduled embryo transfer, freezing all the embryos and delaying the transfer is preferred. If the uterine lining is not of adequate size or fluid is present this is going to impact the pregnancy rate. Endometrial receptivity is one of the main factors that can influence successful pregnancy rates following IVF treatment.

Luteal phase defect

A luteal phase defect can happen in cases where the development of follicles is poor and it takes an extended amount of stimulation to get the follicles to develop properly. The luteal phase is the time in between ovulation and the confirmation of pregnancy or menstruation. Progesterone is  secreted or produced during the luteal phase of the cycle and plays a significant role in preparing the uterine lining for embryo implantation. If the progesterone does not prepare the uterine lining it would be better to freeze the embryos, and transfer in a different cycle where the lining is prepared with an adequate amount of progesterone.