When you’re considering fertility preservation, a common question is whether to freeze eggs or embryos. These two forms of assisted reproduction technology have many similarities, but there are also differences.
Egg versus embryo collection
Both procedures involve stimulating the ovaries to produce multiple follicles, which contain the eggs. When an ultrasound shows that the ovarian follicles are the right size, the egg collection will be scheduled. This involves inserting a needle near the ovaries to collect the oocytes.
If you use egg freezing, the oocytes will be soaked in protective chemicals and then frozen.
If you will be storing embryos, then after oocyte collection comes fertilization. Your partner or sperm donor can come to the fertility clinic with you and provide a sperm sample on the day of the egg collection. Alternately, a frozen sample can be thawed. Classic in vitro fertilization (IVF) places sperm and egg on the same laboratory plate and allows fertilization to occur. However if your partner has low sperm count, your doctor may recommend intracytoplasmic sperm injection (ICSI) instead. ICSI cleans and filters the sperm and injects a single sperm into the egg. After IVF or ICSI, the fertilized embryos will be grown for 3 to 5 days, and then frozen. For more info read The IVF Process.
Using the eggs or embryos
Frozen oocytes or embryos can be kept preserved for years. When it is time to use them, it is important that your uterus is receptive to an embryo implanting, as it is after ovulation. You can use lower doses of fertility medications to induce ovulation, or your doctor can monitor for when ovulation occurs naturally. At the right time in your cycle, one or two embryos will be transferred into your uterus.
If you use embryo freezing, the embryos will be warmed on the day of the transfer. If you use egg freezing, the eggs will be warmed and then fertilized using ICSI.
Nowadays most eggs and embryos are preserved using vitrification. This process liquid nitrogen to cool the cells extremely quickly. To allow for this rapid cooling, higher doses of protective chemicals are used, similar to antifreeze. The water molecules slow down so fast, they don’t have time to form ice crystals, which could damage the eggs or embryos.
Around 95% of vitrified embryos survive the warming process and can be transferred.
Many people still believe that oocytes do not survive freezing and thawing well. While this was true of the older slow-freezing method, oocytes survive vitrification much better. Between 85% and 95% of vitrified oocytes can be successfully re-warmed.
Once they are successfully warmed, previously vitrified oocytes are have fertilization rates and pregnancy rates that are just as good as fresh oocytes.
Choosing between eggs and embryos
Freezing eggs has advantages over freezing embryos. Cryopreserved oocytes are fertilized at the time you use them, rather than when you freeze them. If you are single, this allows you additional time to find a partner or husband. It gives you additional freedom to make a baby with your partner at the time you are both ready.
Previously, women had to choose between higher thawing success rates with embryos, and lower success with oocytes. However, the warming success rates for vitrification are almost as good for eggs as for embryos.
Freezing eggs allows for the freedom to choose the baby’s father at the time you will become pregnant. On the other hand, if you are already certain of your partner, frozen embryos still have slightly higher survival. Knowing these differences can help you choose the best procedure for you.