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Is It a Miracle?

Billions of women fall pregnant through natural around the world but many would say that when looking at the complicated journey of the egg and sperm meeting to become an embryo and then implanting successfully in the uterus is nothing beyond a miracle!! Lets look at the different intricate steps that are involved.

Fertilization and Implantation Steps

Step 1 - Ovulation

Women have 2 fallopian tubes, 2 ovaries and follicles within the ovaries.  Each ovulation cycle if successful a woman will produce one healthy ovum (egg) from one of their follicles within one of the ovaries which must be picked up by a healthy fallopian tube on that side of the ovary.

Usually it is only 1 egg released by one of the ovaries but occasionally 2 eggs can be released from one ovary or 1 egg can be released from each ovary and this can then result in multiples if more than one egg fertilises.

Step 2 - Successful Sex at the Right Time

The man and woman need to make love successfully (ie. sperm ejaculated into the vagina) at the right time in the Fertile window which is 24 hours after release of the egg (ovulation) and up to 5 days before release of the egg.    As sperm live up to 48-72 hours this means that if you have sex before the egg is released it can still be fertilised up to 48-72 hours after by sperm that reach the fallopian tube.

Step 3 - Sperm Making a Successful Journey

There are a number of steps that the sperm need to make on their journey to reach the fallopian tube.   They need to successfully bypass the cervical membrane folds which extend through the cervical canal, then swim through the uterus and along the fallopian tube towards the released egg.  Uterine muscle contractions help the sperm move towards the fallopian tubes.   Fast swimmers an reach the egg within half an hour while others may take days.  Only a few hundred of these sperm will manage to get close to the egg due to many natural barriers such as low pH in the vagina and cervical mucus such as cervical mucus that exist in the womans body along the way.

The sperm need to be able to bypass the cervical membranes then pass through the uterus and along the fallopian tube towards the released egg.  Millions of sperm are ejaculated but only 100’s will likely make it into the fallopian tubes.

Step 4 - Fertilisation in the Fallopian Tube

Gentle muscle contractions will push the egg along the fallopian tube.   On its journey down the fallopian tube just 1 sperm needs to be able to penetrate the outer layer of follicle cells of the egg and then bind to an cross the egg coat (zona pellucida) then finally bind to and fuse with the egg plasma membrane which will result in fertilisation of the egg and if this happens the nucleus of the egg and the nucleus of the sperm will join together to forma Zygote.  

If none of the sperm manage to fertilise the egg then the egg will be continue to be pushed gently into the uterus and will be expelled from the body in the next menstruation.

Step 5 - Zygote to Morula

If an egg develops properly it will split into 2 cells, then 4 cells then 8 cells and a good quality egg will have cells that are of a symmetrical shape.  When it splits again into 16 cells it will become a Morula.  

Step 6 - Morula to Blastocyst

As the Morula continues to develop there will be fluid that starts to accumulate forming a cavity called the Blastocel and this then gives rise to a Blastocyst

Step 7 - Implantation

Implantation is an intricate process and crucial to allow the Blastocyst often now termed an Embryo to continue developing and grown into being your Baby.  The embryo will bury itself into the lining of your uterus which will trigger the placenta to begin forming.  The placenta will then secrete hCG (Human Chorionic Gonadotrophin) into your blood and urine.   Once the quantity of hCG builds up enough over the next 11 days there will then be enough in your urine for a Pregnancy test to detect and that is when you see a Positive Result!

Alberts B, Johnson A and Lewis J et al (2002) Molecular Biology of the Cell. 4th Edition. 

Miller D (2018) The Epic Journey of Sperm through the Female Reproductive Tract, Animal 2018 12 (Suppl 1), 110-120

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