Human fertilization and early development

 When sperm wants to go to the egg, it is actually one of several 100,000,000,000 sperm cells that had a race to see who would get to the female egg first, as when you are born these many sperms would race to see who would be the one that combination of DNA from your father.

A sperm cell is a gamete from your father, and an egg cell is a gamete from your mother.
Each gamete has 23 chromosomes, not chromosome pairs, but chromosomes.
Once the egg is fertilized, the 23 chromosomes form the 46 chromosomes or 23 paired chromasomes.

A Zygote is a fertilised egg, which has 46 chromosomes is made out of two cells or nuclei, but these aren't fully fused yet so they are named pronuclei, which is where most of the genetic makeup of your father and your mother is given to construct you.

Once this happens the cells start dividing, splitting into 2, then 4, and you get a cell division every couple of days. Until you have 16 cells named morula which after 5-9 days, the morula keeps splitting and now has around 200-300 cells, which is now named a Blastocyst.

Doctors and scientists use a table which plots the average childbirth process. But they always start on week 2, why week 2?.
It is because of a gestational age. Which starts when the mother has her last menstrual cycle. Note that this could be 2 weeks before the day of conception of the embryo.

At week 2, starting the embryo stage, which starts the human embryo at the size of a blueberry.
Appearing 7 weeks after the conception of the embryo. Most would call this a fetus. Being around 5cm in size, the boundary between embryo and fetus is not really well defined, but most if not all believe that after the 12th week, it is a fetus.

Around the 42nd to the 46th week or the 9th month is when the baby is ready to be extracted by the mother. 

But the 27th to the 30th week or 7th month has a viable survival chance if the baby were to come out of the womb, it would survive, but most wait for the 42nd week.

By the 5th month or the 23rd to 26th week, if the baby were to come out it would have a fifty per cent survival chance.