I had two Endos, a high risk OB, and an MFM explain the mechanism for Hashimoto's attack on a fetus to me. Their explanation as I interpret it is that in patients with Graves and Hashimoto's the antibodies of the mother cross the placenta barrier and attack the thyroid of the child. This occurs when the mother is not balanced in thyroid levels or monitored properly. The increased risk of miscarriage results from damage caused by thyroid attack on the developing fetus. Hashimoto's tends to be worse than Graves initially because there is too little hormone for proper development.
In the first 12 weeks fundamental systems are developing if ones antibodies are not under control and there is not enough T4 for the developing infant than the fetus may not develop properly and be rejected by your body. Increased antibodies with adequate levels of T4 should allow for proper development, while the developing fetus must be monitored for the development of nodules and thyroid growth with frequent targeted ultrasound and test for hypothyroidism upon delivery. In the case of Graves antibodies you have to fight hyperthyroid issues in the child and they add other complications later in pregnancy.
I have not had a miscarriage that I know of I have only been pregnant once after being told I was infertile. The pregnancy was rough on me, but my child came out fine. He was a 10 lb premie.
Big boy. I am at a high risk now because my antibody levels climbed above 1000 and my T4 levels are in the out house. I am using selenium in an attempt to lower my antibody levels and treating my hypoadrenalism, hypoglycemia, AND Hashimoto's and Graves. I am following my MFM's advice get my T4 levels above 50%, get my TPOAb's and TGAb's as low as I can and then go for it. As soon as I suspect pregnancy get my FT levels checked and up my T4 intake and test FTs every 2-4 weeks and adjust accordingly. Targeted ultrasounds will be performed ever 8 weeks. I posted a thread after talking with my MFM and Obs. I will dig it up for you in case you are interested.
So I can not say what was the cause. Some other stat's on pregnancy in general: only 25% of pregnancies actually take, only 25 % of multiple pregnancies result in a multiple birth with extra fetal matter being reabsorbed to strengthen the remaining fetus. I found out that left handedness is suspected to be the result of the survival of the mirror image identical twin. Having an Ob in the family WHO loves to teach and research gets me all sorts of neat information. Most miscarriages occur before we know it. They are often mistaken as missed/late with a heavy period. I have had a few of those. Finally if a hypothyroid patient gets their T4 levels in the 60-80% range of normal the chances for cognitive dysfunction in the developing fetus is no greater than that of a woman with a normal functioning thyroid. Women with antibodies are at a higher risk for miscarriages, but my SIL OB and MFM and high risk OB have had HUNDREDS of patients with Hashimoto's deliver happy healthy babies. My SIL reminded me of that last night. One can not give up hope, what will be will be.
I can only wish you the best of luck. My husband and I started fighting for treatment of my Hashi's in a pursuit of trying to conceive or second child. Now I have MANY conditions to deal with and we as still not ready. However we are not giving up. We will race the proverbial clock. If I hit 35 before my levels are stable, then a second child was not meant for me. You can only do your best to prepare yourself and then go with your gut and heart to determine if you are ready to take the risks. In the mean time I bless my lucky stars for my son and will do the best by him I can.