IVF lab procedure

Oocyte recovery

After the fluid is removed from the follicles, an embryologist looks for oocytes in the fluid under a microscope. When an oocyte is found, it is picked up in a very small amount of fluid, washed, and placed in the incubator. Oocytes are often found in 75 out of 100 of the follicles over the size of 10 mm. About 80 out of 100 oocytes will have finished maturation by the time the lab does insemination. The example below is based on our program’s average oocyte recovery, maturity, and fertilization rate. This can change from person-to-person and cycle-to-cycle.

Oocyte recovery and fertilization rate

16 follicles over 10 mm X 75% recovered = 12 oocytes
12 oocytes recovered X 80% mature = 10 mature oocytes
10 mature oocyte X 70% fertilize = 7 embryos

Mature oocyte

Figure 1. The mature oocyte. The oocyte has 2 surfaces that must be penetrated by sperm.  The 1st is the zona pellucida. The zona pellucida protets the oocyte and stops extra sperm from getting in the oocyte after one sperm has penetrated it. The plasma membrance is the 2nd surface. The polar body is extra genetic material the oocyte sheds before fertilization. A polar body is a sign of oocyte maturity A 2nd polar body is shed after fertilization.


Oocytes are inseminated using conventional in vitro insemination techniques or intracytoplasmic sperm injection (ICSI). 

  • Conventional (routine) in vitro insemination is the incubation of the oocytes and sperm overnight. 
  • ICSI injects a single, live sperm into a mature oocyte. ICSI is needed for couples who have no or very low rates of fertilization during other cycles. It may also be needed when the number of normal motile sperm is less than that required for routine IVF. The care team will talk with you more about ICSI if it is needed. 

Oocytes are inseminated about 5 hours after retrieval. After insemination, they are left undisturbed in the incubator for 12 to 18 hours. The 5-hours between retrieval and insemination mimic the time needed for the sperm and oocyte to travel and meet in the fallopian tube after intercourse and ovulation. No matter the insemination method used, about 70 out of 100 mature eggs will have normal fertilization.  

Confirmation of fertilization

The oocytes are checked for signs of fertilization 12 to 18 hours after insemination. A normally fertilized oocyte will have 2 large structures inside the egg called pronuclei (each structure termed a pronucleus).

Normal fertilized oocyteFigure 2. A normally fertilized oocyte. It is also known as a pronuclear stage embryo or zygote. The 2 structures in the middle of the oocyte are called pronuclei. One pronucleus has the genetic matierial from the male partner's sperm. The other pronuclear has the genetic material form the oocyte.



Embryo culture

IVF and embryo culture involve making the conditions in the lab as close as possible to those in the female reproductive tract. Embryologists in the lab control the:

  • Nutrients
  • Acidity
  • Humidity
  • Temperature
  • Gas composition of air
  • Exposure to light 

Embryos are transferred on day 5 after retrieval (blastocyst stage). 

Pronuclear Stage
Pronuclear Stage 1 day after retrieval
4-Cell Stage
4-Cell Stage 2 days after retrieval
Blastocyst 5. days after retrieval
Blastocyst 5. days after retrieval
Hatching Blastocyst
Hatching Blastocyst 5.5 to 6 days after retrieval

Transfer day decisions and embryo selection criteria

The goal of IVF and embryo culture is to transfer high quality embryos that can develop normally and result in live births. Optimal culture conditions and reliable embryo selection criteria are needed to choose embryo(s) with the highest chance of leading to a pregnancy.

At UI Health care, about 25 out of 100 pronuclear stage embryos will develop into good or excellent quality blastocysts by day 5 post-retrieval (see figure 3). Embryo selection is based on the rate of embryo development and embryo morphology (how it looks).

Last reviewed: 
November 2021

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