Twins are two individuals who share the same uterus during a single pregnancy, and usually are born on the same day. Due to the limited space in the womb, twins are usually born earlier and smaller than non-twins. On average, a twin pregnancy lasts 34 to 36 weeks. Because early childhood babies often need extra care, twin pregnancies are monitored extensively.
Types of twins
There are five variations in twins. The three most common are all twofold twins.
Chances of twins
The chance of twins is slightly increased by to the following factors:
IVF and twins
Women who receive certain fertility treatments may be more likely to get multiple babies. It depends on what fertility treatment the woman receives. In vitro fertilization (IVF) occurs mainly by multiple embryos being placed in the uterus. Other methods can encourage a woman to produce more than one egg, which can also lead to multiples. Many other treatments have no effect on the number of children.
From angelababy In Q&A