You need to be on synthroid (or a similar med) as soon as possible with a starting dose to be determined by an endocrinologist or OBGYN but definitely not less than 100mcg/day (a current study on this starts women at 150mcg). This is safe during pregnancy.
There is an increased risk of miscarriage, pre-term labor, high birth weight and some evidence of neurologic (eg IQ) development problems. But starting meds now can result in a normal pregnancy.
Also, with the history of postpartum thyroiditis, and the current hypothyroidism you likely have hashimotos and will be on synthroid (or a similar med) long-term.