Answer:
The Halacha is that one may not purchase nor wear new clothes during the 9 days unless it would cause financial loss or one has nothing else to wear. Therefore, if a pregnant woman's figure would change suddenly and dramatically rendering all her existing clothes unwearable she would be allowed to buy new ones. However, since the change is usually gradual she should anticipate and buy the clothes in advance.
One may not purchase used clothing only if it gives him great pleasure and is a source of excitement, otherwise it is permitted.
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