How does conception occur?

Women have two ovaries, which are located in the pelvis on each side of the womb. The main function of the ovary is to grow and release eggs and produce hormones.

For a pregnancy to happen an egg needs to be fertilised by a sperm. The fertilised egg (embryo) then needs to implant itself into the lining of the womb so that it can obtain nutrients to aid its growth.

A number of steps need to take place for this process to occur successfully:

  • The egg needs to grow and mature in the ovary.
  • The egg needs to be released from the ovary (ovulation). Eggs are tiny and have to be magnified hundreds of times with a microscope to be seen by the human eye.
  • The egg needs to be collected into the end of the fallopian tube. The tube achieves this by it’s end moving down to the ovary to the area where the egg is releasing. The inside of the tube is wet (like the inside of our mouths) and has delicate hairs and muscles that gently create a current of fluid moving towards the womb, just like a gentle vacuum. There are finger-like structures (fimbriae) at the end of the tube that catch the egg and it is sucked up into the tube.
  • The egg and sperm need to meet. Therefore sexual intercourse needs to occur around the same time as an egg has been released from the ovary.
  • The egg needs to be good quality
  • The sperm need to be present in good numbers with the ability to swim forwards. They swim through the cervix (neck of the womb), through the womb and into both tubes, to look for an egg.
  • At the same time, the lining of the womb should be thickening up and preparing itself for the fertilised egg to implant.
  • When the sperms find the egg they need to bind to the egg. One sperm will then get inside the egg and fertilisation occurs.
  • Once successful fertilisation has occurred, the egg changes to grow into an embryo. As the embryo divides from two to four cells and then more, it starts it’s journey down the fallopian tube towards the womb. The embryo will eventually become the billions of cells that we are made up of.
  • Once the embryo reaches the womb it begins to burrow into the lining (implantation) and a week or so later there will be pregnancy hormone detectable in the woman’s system and a pregnancy test will be positive.
  • Anything that interferes with this process can affect a couple’s fertility. For example:
  • An egg is not produced. This is due to either very low or absent egg levels or the eggs not releasing (anovulation)
  • The egg is poor quality
  • Sexual problems meaning no sperm are delivered into the vagina
  • Blockage in the tubes allowing the sperm to move from the testicle to the tip of the penis, so no sperm reach the vagina
  • The end of the tube cannot move to collect the egg due to scar tissue inside the pelvis
  • The sperm cannot reach the egg because of tubal blockage or tubal damage, e.g. from infection or pelvic disease, or because the sperm cannot swim well or are present only in low numbers
  • The sperm cannot reach the egg because the sperm cannot swim well or are present only in low numbers
  • The sperm cannot fertilise the egg, due to a problem with being unable to bind to the egg or due to a genetic problem with the egg or sperm when they meet.
  • The tube cannot catch the egg due to the absence of the finger-like ends (fimbriae) or damage to the hairs or muscle in the tube
  • The embryo cannot implant due to problems in the womb
  • The embryo does not implant due to a failure of the cells to grow
  • If the egg is not fertilised the lining of the womb is shed and the blood released in a period or menstruation. This usually occurs 14 days after ovulation.