Taken from www.bith.com.au:
Some women will try and express colostrum from their breasts during pregnancy. If you do it is normal to only to get a few drops out at any one time. This can be done by massaging the breast down to the nipple and then compressing the areola, with your fingers at least a couple of centimetres behind the actual nipple. Some parents express concerns that if they leak colostrum they will not have 'enough' for their new baby when he or she is born. This is a myth, because the breasts are capable of constantly replenishing colostrum and breast milk every 3 to 4 hours or so. Your new baby will get all they need! This is why women can still breastfeed other children while they are pregnant, and a few will even 'tandem feed' (that is feeding the older sibling and new baby) after the birth
Hope this Helps