Gestational hypothyroidism, or underactive thyroid during pregnancy, is a serious issue. Research suggests that an underactive thyroid during pregnancy can cause a range of medical complications for women and their babies. For mothers, these include miscarriage and diabetes in pregnancy. For infants, the condition is associated with low-birth weight and potentially lower IQ.A: Gestational hypothyroidism, or underactive thyroid during pregnancy, is a serious issue. Research suggests that an underactive thyroid during pregnancy can cause a range of medical complications for women and their babies. For mothers, these include miscarriage and diabetes in pregnancy. For infants, the condition is associated with low-birth weight and potentially lower IQ.A: Gestational hypothyroidism, or underactive thyroid during pregnancy, is a serious issue. Research suggests that an underactive thyroid during pregnancy can cause a range of medical complications for women and their babies. For mothers, these include miscarriage and diabetes in pregnancy. For infants, the condition is associated with low-birth weight and potentially lower IQ.