Early in the pregnancy you will not be able to tell that your gerbil is pregnant. The babies are still too small to be noticeable. About day 18, you may notice that your gerbil is looking a bit "bulky". If she is currently nursing, however, this slight change may not be noticeable because her body hasn't gotten the chance to return to a pre-pregnancy state.
At about day 20-23, your gerbil will begin to look more pregnant. If you still are questioning whether she is pregnant, you can usual tell by the "hand test". This just involves setting the gerbil on your hand and seeing if her belly brushes your palm. In the normal, non-pregant state, her belly will be lifted slightly off your hand. When she is pregnant, however, you can definitely feel the light weight of her stomach touching your palm.
Very late in the pregnancy, day 24-28, your gerbil will feel very large and look pear-shaped. There is no question that she is pregnant at this point! Some people say that the female feels lumpy, like a little sack of potatoes. I've never noticed this, but you may. Whatever you do do not press on her belly to "feel" the babies. This is very uncomfortable for your gerbil and cannot be good for the developing pups. Please don't count your chickens before they've hatched.
If she has, you wont be able to tell she is pregnant until late in the pregnancy, I only found out my gerbil was pregnant the day before she gave birth. If so then clean her tank out as you cannot clean it out after the babies are born only about 2-3 weeks after. Make sure the female is not with another female , and do not seperate her from the male until after the next litter then put one of the female babies from this litter with mum to help her raise the babies then she wont have lots more, is she is on her own it could be stressful for her, don't put her with another female her age, as that female may eat the mums babies!! Hope this helps but take my advice, do make sure there is lots of food and the hand test is very reliable!! Try not to touch the babies and drape a towel over half the tank so it is less stressful and she feels safe ;] its to help momma gerbil but don't worry she will be a fab mum. The dad wont eat the babies, except in very rare situations, the female is most likely to eat the females so don't worry, lol!!!!
Hope that Helped!!
Hope I've helped !!