Most over-the-counter medications, like antihistamines, decongestants and cough medicines are not recommended for pregnant women, simply because many of them have not been tested on pregnant women and there is no telling what a certain medication can do the baby. For most of those medications the negative effects would probably not be extremely dangerous but it is certainly better to be safe than sorry, especially if the illness is just a common cold that will go away soon on its own.
Pregnant women should especially stay off ibuprofen and Aspirin. Some Tylenol is probably fine for minor aches.
In cases of complications which usually require antibiotics, like sinusitis or pneumonia, pregnant women must consult their doctor. He/she will determine if antibiotics are allowed for that particular case. A women should never take antibiotics without consulting a doctor, especially leftover antibiotics from a previous illness.
Common cold and flu can be treated effectively at home, using natural remedies and very important thing is to get plenty of rest. The immune system needs rest in order to be able to fight off the infection.
Drinking plenty of fluids is another essential thing. Water is probably the best, but soups, herbal teas and freshly squeezed juices are also very beneficial. Fluids help with congestion and dissolve the thick mucus in sinuses but also in lungs.
The diet should be healthy and well-balanced, with lots of fresh fruit and vegetables. Food should be light, not deep-fried, greasy, or very spicy.
Steam inhalation, using water with essential oils (but not lavender or rosemary, which are not suitable for pregnant women), can help with nasal congestion and sore throat.
One of the best ways to cure a sore throat is to gargle saline solution several times a day. A saline solution can be made with one cup of warm water with half a teaspoon of sea salt.
Peppermint tea is also good, and it helps eliminate the unpleasant smell in the mouth which occurs sometimes during a cold.
Sage tea is also excellent for gargling, because it has antibacterial properties.
Honey, lemon juice and a teaspoon of vinegar make a good syrup for sore throat. It should be warmed up on the stove and sipped slowly.