What is fertilization?
In this series of guest blogs, Dr Grace Petkovic takes us through some of the basics of reproduction and reproductive health.
Grace is a junior doctor in Liverpool, England with an interest in reproductive endrocrinology.
How do we get to fertilization?
Conception and fertilization are interchangeable terms that describe when a sperm and an egg meet and fuse to form a new organism.
Natural fertilization happens during sexual intercourse when a man ejaculates semen, containing sperm, into a woman’s vagina. The sperm travel through the vagina and, if conditions are right, enter the cervix, which is the gateway to the uterus (‘womb’). The sperm keeps moving, towards the fallopian tubes which are attached to the uterus.
A healthy woman will release an egg once a month from her ovaries. This is the day of ovulation. This ovulated egg enters the fallopian tubes. The time between ovulation (egg release) and menstruation (period) is usually 14 days. This means a woman with a 28-day cycle, can expect to ovulate around day 14.
Summary Table: (Approximate) dates in a woman’s menstrual cycle.
Within a woman’s monthly cycle, she has a ‘fertile window’ of about 6 days. This is the time during which she may become pregnant if she has sex. This window is usually the 5 days prior to ovulation and the day of ovulation. Sperm can live about 5 days in the woman’s body [1]. If a man and woman have sex in this fertile window then the sperm may be alive ready to fertilise the egg, even if the egg has not yet been released. Fertilisation (sperm fusing with egg) occurs in the fallopian tube.
Artificial fertilization is sometimes conducted in a laboratory. This is known as In Vitro Fertilization (IVF). In Vitro is Latin for ‘in glass’. This refers to a ‘glass’ dish traditionally used in laboratories. For couples who cannot conceive naturally, eggs from the woman and sperm from the man are extracted. They are then mixed ‘in vitro’ to assist the couple to conceive. If fertilisation is successful, the new organism is transferred to the woman’s uterus.
What actually happens at fertilization?
The membranes of the sperm and egg fuse, creating a new single cell. After fertilization, there are rapid chemical changes in this new cell. This new cell internally directs its own development and communicates with the mother.
From single cell stage, until about 2 weeks after fertilization this newly conceived human is called a zygote. It continually divides and develops.
Embryo, Zygote, Baby. Why do people use different terms?
Just as those over 18-years-old we call adults, and those under-18 years-old, we call children, so we have different terms at different stages of gestation. Let’s cover some basics [2,3]:
Zygote – a term used up to 2 weeks after fertilization, before implantation. This zygote needs to attach itself (‘implant’) into the lining of the uterus.
Embryo – a term used once the organism has implanted into the uterus and the placenta starts to form. Implantation occurs approximately 1-2 weeks after fertilization. The placenta is a shared organ between mother and child, allowing transfer of substances between their blood streams.
A lot of people use the word ‘embryo’ loosely to refer to the developing organism between fertilization and second trimester.
‘Baby’ is not a scientific term. There is no strict age criterion for when someone would qualify as a ‘baby’. Many people use the word ‘baby’ as soon as they are pregnant.
References and footnotes
[1] Remember that ovulation only roughly happens on ‘day 14’ so avoiding sex ‘roughly around day 14’ is not a reliable method of avoiding becoming pregnant! This is different from other methods that calculate (rather than estimate) when ovulation is happening, or has happened, and use that to guide when a couple have sex, or not have sex in hopes of achieving or preventing pregnancy.
[2] https://www.medicinenet.com/embryo_vs_fetus_differences_week-by-week/article.htm#what_is_an_embryo
[3] https://embryology.med.unsw.edu.au/embryology/index.php/Embryonic_Development