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Frozen embryos is a common way for couples to maximise the chance of falling pregnant from a single IVF or ICSI cycle. When using frozen embryos, there is a process of thawing the embryos and then transferring them to the uterus. There are three ways in which a uterus is prepared for the transfer of a thawed frozen embryo. 

1. Natural menstrual cycle  

If you’re ovulating regularly, the embryo can be transferred naturally. To know when the embryo should be transferred, we need to track development of your eggs, the thickness of your endometrium lining (lining of the uterus) and your hormone levels. 

A vaginal ultrasound allows us to track the development of your eggs and measure the thickness of your endometrium lining. To test hormone levels, a blood test is required. Providing that the follicle size, endometrial thickness and hormone levels are normal, your embryo can be transferred. 

Your embryo is transferred to the uterus an appropriate number of days later (six days later if a blastocyst is to be transferred or four days later if a day 3 embryo is to be transferred). If you have intercourse during the cycle, there is a chance of natural pregnancy in addition to the chance of success from the embryo transfer.

2. FSH ovulation induction

If you’re not ovulating regularly and have sufficient eggs in the ovary to ovulate, FSH ovulation induction would be suited for you.

FSH is a hormone injection that stimulates the growth of more than one egg. If two or more eggs develop in the ovary, you may be asked to avoid sexual intercourse because of the risk of a multiple pregnancy. Like natural cycle FET there is a risk of twinning if natural conception is able to occur. Ovulation may be induced by the injection of hCG.

3. Hormone replacement cycle.  

If you’re not ovulating regularly, have a reduced number of quality eggs, or have problems with endometrial thickening, hormone replacement cycle might be best suited for you.

Sometimes a GnRH agonist (Syneral) has to be used to prevent egg development from the ovary interfering with the hormone levels, which affect the endometrium lining. To help thicken the endometrium lining, a table or patch is applied to the skin every three days. Once the endometrium lining is the appropriate thickness, a second hormone, called progesterone is administered vaginally (Utrogestan, Endometrin or Crinone). 

 The embryo is transferred to the uterus at the appropriate time after the progesterone is commenced. Pregnancy tests via blood tests are required and if the test is positive, the hormones must be continued until about eight or nine weeks of pregnancy by which time the placenta produces enough hormones to keep the pregnancy in place. The hormone supplements are then gradually reduced after ten weeks of pregnancy.