The babies, by now, have got their organs and systems functioning normally with the exception of the lungs. The lungs will continue to grow until they are born. As they continue to put on more weight in the final lap of confinement, you may get completely bogged down. But let this aspect not get you worried. The rest that you take is the most important aspect of their life and yours as well.
Keep following the advice of your doctor and consume the nutritional diet and the supplements. By now, the heads of your babies are in proportion to their bodies. The uterus is pushing the diaphragm for space. Whenever you sit up, do keep your back straight and if you lie down, do keep a pillow under your feet so that your back does not hurt.
The visits to the doctor would become more frequent during the last few weeks. In one of these visits, do not hesitate to ask the doctor again the difference between the Braxton Hicks contractions and preterm labour. In any case, he would be checking out the extent of dilation of the cervix while doing the pelvic exam.
But, the bottom line at this stage is simply this that you should rest as much as possible. Yes, you will still feel breathless, you may not feel like eating much and every movement would be difficult. There is nothing really to worry because it is the sheer weight of your babies that is preventing you from eating or your back ache not getting relief.
At the end of it all, you will have the pleasure of seeing them. You have carried on for so long. A little patience will see you through this wonderful journey of pregnancy.