The first trimester of pregnancy is synonymous with nausea, heartburn, and a lot of discomforts. As you head forward, together with hormonal changes, your body also changes a lot. So, wearing clothes that are comfortable and can also expand with your changing body is very important.
As your pregnancy progresses, your regular nighty may not fit anymore, or at least not as comfy as earlier it used to be. That's the time when you would want to switch from your regular nighty to a maternity nighty. It would help if you have nightwear that is comfortable, relaxed fit, soft on the skin, and grows with you.