Thousands of sperm are released in a single ejaculation, but many don’t even make it close to the egg. Although hundreds of sperm may surround the egg, only one can successfully penetrate it, then the egg changes, so that no other sperm can enter. The single-cell zygote, then transforms into a multi-celled embryo. That embryo then needs to move down the fallopian tube and implant in the uterus, in order for a successful pregnancy to occur. The conditions have to be just right, for conception to happen.

For many couples or individuals who are unable to conceive naturally, ART (Assisted Reproductive Technologies) gives them the hope of having their own child, biologically linked either to them, their partner or both of them.  

There are several different ART procedures available, including IUI (intrauterine insemination), IVF (invitro fertilisation) and IVF with ICSI (intracytoplasmic sperm injection). The difference between the three procedures, is the method used to allow fertilisation to take place.

How does fertilisation take place with IUI?

IUI is often used for couples or individuals who have been unable to conceive, but where there is no identifiable barrier to conception. It is popular with the LGBTQ community and individuals without a partner, as it’s generally a low-risk procedure, less invasive, with few side-effects and costs less too.  

With IUI, there is no need for egg collection, as the sperm is placed directly into the uterus, via a thin tube, for fertilisation to take place in the same way as it would with natural conception. The female partner’s cycle is monitored, to pinpoint the best time for this to happen.

It’s possible to have a stimulated cycle (with fertility drugs) or an unstimulated cycle (without fertility drugs).

How are the eggs collected for IVF and ICSI?

The process usually starts with fertility drugs, administered at home, via self-injection. This stimulates the ovaries to release more eggs than usual. Scans, or blood tests can detect how the follicles are responding, to work out the best time for egg collection.

With the female partner under sedation, the eggs are then collected, using a fine needle passed through the vaginal wall.

More medication may then be used to prepare the lining of the uterus ready to receive any viable embryos.

How does fertilisation take place with IVF?

The healthiest sperm are selected and placed in a dish with the eggs in the lab, to allow fertilisation to take place.

Experts will monitor the development of any viable embryos.

How does fertilisation take place with ICSI?

The healthiest sperm are selected and one sperm is injected directly into each egg. After this, the procedure is the same as for IVF, where experts will monitor the development of any viable embryos. ICSI is often used when the barrier to conception appears to be caused by male factor infertility.    

At the IVF Network, we know that individuals and couples who are trying to conceive are likely to have many questions. Through our dedicated channel of experts, our website and our blog posts, we provide information to help to answer some of the more frequently asked questions, to support you on your fertility journey.


Conception: From Egg to Embryo Slideshow

Grow, by Web MD   Reviewed November 2021

Burch, K. August 2022   Medically reviewed by Schlanger, L. MD

Very Well Health

What Is Conception?

The Fertilization Process From Ovulation to Implantation