Help me understand my mum.

My mum and I, we have an excellent relationship and we are four siblings. I am currently 9 weeks along, and my mum insists upon absolutely no outdoor trips outside of my necessary commute to and from work. This weekend, my husband and I hit 2 stores in a car and the whole trip took about an hour and a half. Before I left to shop, my mum tried to convince me to let my husband go alone, failing which she asked for my assurance that I will only go to the 2 stores planned and will not detour elsewhere. My mum has has four children and his is my first. Is my mum's concern warranted? Should I be minimizing my activity? If do, for how long?