I have read the same, related to low estrogen & progesterone levels but I don't think this should stop you from trying. It also depends how much breastfeeding your doing, your daughter I'm sure is doing solids also and is not full time nursing. I would not wean my baby for that reason alone, unless the timing is right for both of you. My son went until 14 months, kudos for you even being ready for another yet! I wanted a little boob freedom before I was ready for my hubby to get near em, i.e. to get prego again! It could be 3 months plus before you get prego anyway, by then you might be done or at least down to a couple of feedings. My girlfriend at 33 nursed up to the end of her pregnancy and planned to tandem nurse but it didn't work for them. I think it is easier the younger you are, but I would not worry about it, our bodies are smarter than we are, go for it! You also might get the La Leche leagues opinion on the issue, I bet they would have another take on it.
K.
35, 6 months prego with a great 2 yr old boy