What triggers hypothyroidism? What is the impact of hypothyroidism on the foetus?

Most of the time, the reason is unknown. Sometimes it's due to having autoimmune disease, where your thyroid is having antibodies attacking it so the hormones drop. Whether the thyroid hormones are high (hyperthyroidism) or low (hypothyroidism), it is not good for the baby and sometimes this abnormality, because of antibodies causing the hormones to be abnormal, the baby can born with thyroid hormone imbalance (because the antibodies are passed on to the baby). This can cause the baby to not try well (also known as "failure to thrive") and they can't sucker well ,they don't feed well, they don't grow well. These things must be discovered immediately at birth or else the baby won't grow well, which may affect their brain and cause mental retardation. As part of the normal practice, most babies are checked for thyroid hormones.