Pregnancy: What is Fertilisation?
Fertilisation is a classic story of sperm meets egg. It’s the first thing that needs to happen for a pregnancy to begin. Let’s take a journey into the fallopian tubes…
When the egg cell is released by the ovary, it moves into the fallopian tube which connects the ovary to the uterus, thanks to the small cilia that line the inside of the fallopian tubes. These incredible little cilia will help the egg cell to reach its final destination. On its way, it may or may not meet the sperm that set out in its pursuit. During this time, the ruptured follicle has remained at the surface of the ovary. It turns into a corpus luteum. This corpus luteum
secretes progesterone as well as small amounts of oestrogen. The joint action of these two hormones contributes to the thickening of the lining of the uterus. If fertilization occurs, it will be snugly implanted and the corpus luteum will continue to produce high levels of progesterone to keep it in its cocoon. The
cocoon will release a hormone — human chorionic gonadotropin (HCG). This is the hormone that’s detected during urine pregnancy tests and is what makes the pregnancy test strip turn positive. If the egg doesn’t become implanted, you’re not pregnant, the progesterone level falls and causes the lining
of the uterus to disintegrate. This takes you back full circle to having your period again.
Worth knowing: the inner lining of the uterus increases in thickness tenfold between the start of your period and the end of the cycle, from 0.5 mm to 5 mm in 28 days (if you’re not pregnant).
The 5 day lifespan of a spermatozoa
After ovulation, the egg cell has a lifespan of 12-24 hours. During this time, it might be fertilised by a sperm cell (spermatozoa). To be that lucky guy, one spermatozoa has to beat the 200-300 million others (yep, that’s how many sperm cells are released by one ejaculation) to reach the egg.
After entering the vagina, the fastest sperm travel up the cervical canal of the uterus. Cervical mucus is produced to ensure survival and improve their agility. Speed is important: sperm are up against the clock – and each other – after 5 days they will die out.
Of the millions of spermatozoa that started the race, only a few hundred make their way into the fallopian tube where the egg is patiently waiting. The best one penetrates the egg cell, triggering the fertilisation process. Once the egg is fertilised, it becomes a zygote.
Because sperm can survive for 5 days in the female reproductive system, there is still a chance of conception when having unprotected sex when you think that you are not ovulating.
Initially, the embryo looks like a blackberry
After fertilisation, the zygote begins to divide very quickly to become an embryo. The zygote becomes a small cluster of cells that look like a blackberry (though it’s much, much, smaller than that). The cluster, called the blastocyte, continues to move through the fallopian tube towards the uterus where it has to embed in the lining of the womb in order to continue development and later become an embryo.
The implantation is defined as the beginning of pregnancy. This usually takes place between 6-12 days after fertilisation.
Essential Information
If you use emergency contraception, like the morning after pill or the copper coil, this can only prevent pregnancy – it cannot interrupt a pregnancy that has already begun.
The morning-after pill works by delaying ovulation, so there is no egg to meet the sperm. If you have already ovulated in the time between unprotected sex and taking the morning after pill, then the emergency contraceptive cannot be effective but it will have no effect on the foetus if you become pregnant.
