Constipation in pregnancy is caused by relaxation of smooth muscle due to the effects of high levels of progesterone. There is just not enough peristalsis.
If it is bad, try something like movicol, until you get things moving, then use diet. If it is really dry and hard and bunged try glycerine suppositories. Fibogel does not work well - as things move through the colon, the fluid is absorbed and the faeces dry out. If it is slow moving, it dries out too much, causing it to slow down and creates a vicious cycle. Fibogel dries up, as does bran, and causes more of a blockage.
Eat a good breakfast of things that help - fruit, prunes, porridge, muesli, whatever. This should encourage peristalsis, and going to the loo. Kiwi fruit are particularly good and contain some enzyme or other that helps. If you eat one every day, you will not have a problem (other than they taste horrible after the first six months 
.) They seem to give a good daily visit to the loo, and without the tummy aches and wind you can get with other things.
Then go to the toilet at the same time every day and try and set up a habit. You need peace and quiet and a foot stool. The suppositories can help here. Do not ignore an urge. A hot drink beforehand is supposed to help.
Eat plenty vegetables, pulses, and oats (soluble fibre) and drink plenty - don' t get thirsty.
You should get movicol on prescription if you have a bad time. Do not be tempted to take anything with ducosate in it as this can give really, really bad cramps in some people. I am not sure it is recommended in pregnancy and it really ought to have a warning on it.