In-vitro fertilisation (or IVF) is the process of fertilising
outside of the body. A literal translation - fertilisation in glass.
This is what people pay a lot of money for when they have problems conceiving by the more orthodox methods.
The female is given a drug to stop ovulation and then another drug to cause her to ovulate, but more than usual, resulting in a "harvest" of ova for treatment. The sperm is obtained from a husband/donor/partner and is then used...
It involves introducing the sperm to an ovum to try and fertilise. Several ova will be fertilised and then implanted into the womb. Then, along with a treatment of fertility drugs, there is a wait to see if the fertilised ova stick to the wall of the womb. If they do (and quite often it is more than one, hence multiple births) then the woman becomes pregnant and, barring any complications, carries a child (or several children) to term.
Statistically, there are now more multiple births due to the amount of fertility treatment that is taken by people throughout the world. However, in many cases, a woman would be advised to have no more than 2 or 3 embryos at once, which occasionally results in partial termination.