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As soon as your existing garments start to feel too tight, probably from about 12 weeks on.
Normally it is advised that you buy your maternity wear in the size you are pre-pregnancy if your bump is the only area where you have changed shape. However, if you are eating more than normal and have changed size on your hips and around your bust be realistic and buy the next size up.
It is recommended that if you are having twins that you also purchase your maternity wear in the next size up from usual. Obviously carrying two babies instead of one means you will get bigger than a singular pregnancy.