i have no great wisdom but i wanted to tell you the EXACT SAME THING!!! happened to my ds!
His first three teeth to come in, the two central upper incisors and the second upper right incisor, all had a yellowish-brownish line across them, and by 9-10 months they were starting to chip. He actually broke off the bottom of one by bumping it on a blanket! the rest of his teeth came in normally. We started brushing his teeth/gums after nursing but hadn't up until that point.
Here's where i have no advice, because the town we were living in had waaaaay too much demand for ped. dentists so it was literally a yearlong waiting list. The first dentist we went to said, "yep, you're right, the enamel is soft" and then "you should see a pediatric dentist" um, i THOUGHT that's what our appt was for LOL
They also were very negative about breastfeeding which i totally ignored since they couldn't even book our appointment properly
Just before we moved out of state when ds was 2, we saw this fabulous family dentist in a tiny neighboring town who said he'd seen quite a few similar cases and couldn't be sure what had caused it as i hadn't taken any medications during pregnancy (such as tetracycline). He was of the opinion that even if it is ever proven breastfeeding is detrimental to some children's teeth the benefits far outweigh the risk, and did not think that had played a part. We were expecting to be told at that time we'd need to pull the teeth, but since ds was not experiencing any pain when biting, did not have any discharge, gum inflammation, or pockets/looseness around the teeth, extraction was not indicated just yet. He did scale off the soft enamel since it was flaking off anyhow, and applied a flouride solution only to the three affected teeth. he did not recommend going overboard with flouride.
We did end up having the two central incisors pulled and the two on either side crowned as the three teeth were basically dead ~ the roots were pretty much gone by the time ds was 2.5 (his first xrays as the previous dentist didn't want to do rads at the first visit), and one had started to abscess. The pediatric dentist we saw here reassured us that genetics and the luck of the draw were factors more than anything else ~ how else to explain kids with kool-aid bottles and only a cavity?
OH MAN i am sorry about that longass story!! I wanted to let you know her babe is not the only one!! The point is that i agree with simonee completely, and all the rest of ds' teeth are fine (although he does have a propensity toward cavities if we so much as blink). NOTHING i could have done while nursing would have caused this extreme of soft enamel/decay! Everything i read at the time also seemed to indicate that the teeth were doomed from the start but it was a matter of waiting until they actually were diseased enough to pull, or waiting for the adults to come in (his adult teeth on x-rays are perfectly normal by the way, even though they were sitting in bacteria from the abscessed tooth!)
I hope you haven't gone blind!
Good luck, i hope you find a more understanding and holistic dentist!! It's really too bad there aren't smilemomma clones! Send one to minnesota asap!