It must be noted that during pregnancy a woman may go under certain changes in her body which can be even annoying for her. But as it is a stage of the changes in the hormonal levels of the body so it must be considered normal. One of the basic changes during pregnancy is discharge from the vagina. So she must take it as a timely change.
The main reason for this is that the vagina gets soft and liable to more secretion as during this critical period of pregnancy there is an increase blood supply to the vagina along with the female hormones named as estrogen and progesterone. So these factors combined with some others can lead to an increased amount of the mucous secretion from the vagina.