In Vitro Fertilization (IVF) is a therapy that is designed to increase the chances of pregnancy for women who have difficulty conceiving naturally and it is the most advanced and effective fertility treatment available. It involves a series of steps that result in the development of embryos that can be transferred to the uterus to achieve pregnancy. Some patients achieve a pregnancy after just one round of IVF while others require multiple cycles. Additional options may be explored for patients who do not respond to IVF, such as using an egg donor.
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Technically, “in vitro fertilization” only refers to the process of fertilizing an egg outside the body. However, the term “IVF” is now used to describe the entire treatment cycle. In natural conception, the fallopian tube provides three functions: it picks up the ovulated egg, it provides an environment in which fertilization can occur and it transports the resulting embryo to the uterus for implantation. These steps can be replaced by IVF in cases where the fallopian tubes are not functional.
IVF is the most technologically advanced and successful form of fertility therapy available. The pregnancy rates associated with IVF are the highest of all fertility treatments across all ages. In a natural cycle, only one egg is ovulated, fertilized and implanted in the uterus. IVF augments these processes by stimulating multiple eggs to grow, fertilizing them directly in the laboratory and placing the most viable embryo or multiple embryos into the uterus.
The IVF cycle has three phases: ovarian stimulation and egg retrieval; fertilization and embryo culture; and embryo transfer and progesterone support. A description of these three main principles follows.
1. Egg Recruitment and Retrieval
A series of hormone injections are administered over a period of two weeks to stimulate the ovaries to produce multiple egg follicles. The egg follicles that develop are the ones that would naturally be lost that month. The eggs are then retrieved in a 20- minute procedure conducted by passing a needle through the vagina into the ovary under anesthesia.
2. Egg Fertilization
Fresh or frozen semen can use for fertilization. Semen is washed and processed, and the most robust sperm selected for fertilization. In general, one sperm is injected into each mature egg to achieve fertilization, a procedure called intracytoplasmic sperm injection (ICSI). Fertilized eggs are then cultured for 3-5 days in the IVF laboratory and the best embryos frozen for future transfer back to the uterus.
3. Embryo Transfer
The embryo transfer is a simple procedure performed by placing a soft guide containing the embryo(s) through the cervix under ultrasound guidance. The procedure is painless. Ultrasound visualization is used to precisely placed the embryo in order to increase the chance of pregnancy.
In most cases, a pregnancy test can be done seven days after the embryo transfer to confirm successful implantation. Progesterone hormone treatment is continued through the first trimester to help maintain the pregnancy.
For a description of the procedure in greater detail, please see The IVF Cycle.
How many embryos are transferred during IVF
The American Society for Reproductive Medicine (ASRM) has created guidelines on the number of embryos to transfer. These guidelines are based on the age of the egg provider and the avoidance of twins or higher order multiples.
In the past, embryos were typically transferred fresh on day 3 of development. To optimize pregnancy rates and limit multiple pregnancies, a guidance of 1-2 embryos for women under 35 years was established. That number rose to a maximum of 5 for women who are older than 40 years of age.
With the advent of improved IVF laboratory conditions, day 5 blastocyst embryo transfer emerged as the standard. Single embryo transfer (SET) is the recommended number in most cases especially when an embryo has been PGT-A tested and found to be chromosomally normal. For women over 35 years of age, transfer of 2-3 blastocyst embryos can be considered especially if not PGT-A tested or if unfavorable factors such as embryo quality or past failed transfers are present.
For healthy women who have been properly counseled on the risks of twins, transfer of 2 blastocysts may be considered.