What happened to Haesoo's 1st pregnancy?
Most likely: She miscarried because of the stress of working alot and debt collectors.
Least likely (to me): abortion.
What I think happened:
Haesoo miscarried and didn't tell Mincheol. I think this is a possibility because in a flashback we see that Haesoo and Mincheol were separated for a bit, it could've happened during that time. We know Haesoo internalizes things and doesn't share her burdens. In general though, with Mincheol feeling guilty for the debt collectors would she share this information too? That's why I think we are shown the doctor appointment regarding her fertility before the miscarriage, both Mincheol and Haesoo are present but I think she went to the doctor appointment alone for the ultrasound, even if Mincheol and Haesoo were together during that time, there are women who wait to inform even their significant other untill the chance of miscarrying is much lower.
Now the only reason I doubt this theory is because the reason I thought Mincheol became trash was because of the debt and the miscarriage. But if he doesn't know about the miscarriage then holy *hit!