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Your estradiol level appears to be fine if pregnancy happened naturally (that is not through treatment like ovarian stimulation). Though studies associate low estradiol levels with viability, the association is indirect (indirectly indicating defective egg formation/folliculogenesis, with resultant defective corpus luteum). Estrogen supplementation to improve viability in natural pregnancy is not studied well.
In fact, it is only the health of embryo, which determines viability (and it cannot be altered through medicines), second hypothesized intervention being progesterone support (which you are already taking).
As other causes like APA, thrombophilia is already ruled out, you do not need medicines for that.
Regarding MTHFR mutation, if your homocysteine levels were high, it is better to take Vitamin B6 and B12 supplements along with Folate. If levels were normal, those supplements are not normally needed. If levels were not checked, you can get it checked or start supplements, as there is no harm in taking water-soluble vitamins.
So, in summary, estrogen supplementation is not known to improve viability/prolong pregnancy, and is not needed. Vitamin B6 and B12 supplementation may be helpful if you had elevated homocysteine levels.
Wish you good luck.