The common cause of growth restriction is reduction in blood supply to the baby through mother's placenta. Though exact cause of this reduction is not known yet, it's thought to be because of complex interplay between immune and tissue factors which lead to defect in the normal formation of the placenta. This formation of placenta occurs early in the gestation, in the first-third month itself, so, the growth restriction is already destined at that early in pregnancy.
So, growth restriction is not due to poor nutrition of the mother or her exertion.
Babies grow slowly till 6 months, during which their organs are formed and shaped. The growth spurt occurs in the last 3 months of pregnancy, so, a restricted baby deviates in growth from a normal baby, after 6 months of gestation. Before this period, the growth restricted (destined to be growth restricted) baby may be differentiated from a normal baby, by doppler ultrasound. But often such diagnosis is not possible, and usually, growth restriction is diagnosed, after it has set in, at 7th or later months.
As the growth restriction is due to supply system of the baby, that's placenta, overloading the mother with nutrients or other things is not going to change the scenario. Placenta is an organ, which sucks in mother's nutrients even across a gradient, to keep the baby nourished at the expense of mother. So, making the mother over-eat is like trying to raise the water level in the upper tank, by raising the level in lower tank, when the motor which pumps it uphill is damaged. It doesn't work.
In fact none of the treatment strategies increase the baby's growth, as seen in rigorous studies.
So, only treatment to be offered to the patient is, continuous monitoring of the baby, and when it's survival in the mother's womb is in danger, to deliver it outside, and treat it after birth.
Totally restricting mother's activities, and confining her to strict bed rest, doesn't help the baby either. Still, the mother may restrain from doing heavy work, and she can do some light routine work.
The mother may keep a watch on her fetal-kicks. Various strategies were devised in measuring these kicks, like count to ten, or count for one hour thrice a day, but these strategies were not proved to be any better than, just casually keeping a note on one's kicks. The mother need not count them. (I personally advise them also, not to compare one's kicks with her neighbour's. Kicks of different women vary, and they vary even in different pregnancies of the same woman, and also in, different months of the same pregnancy.)
Maternal weight measurement have not been proved to be of much benefit. Now we have measures to directly assess fetal weight, health, and blood supply. So, indirectly assessing fetal weight, by weighing mother is not necessary.
In the end, I would like to stress that, babies are normal, only that their nutrition is affected. Once delivered, they grow normally, catch up with rest of the babies, and live like them. Sir Isaac Newton weighed only 2 kgs at birth (normal is 2.5 kgs at the least) and he's considered greatest scientist ever.