A-chan tells Koharu that she should be calling her "Mom".
Which is something an adoptive mother might well say to a kid. It's pretty much impossible for her to have had Koharu and be a top student who graduated with hopes of becoming a doctor, and without Rinko noticing.
If I'm correct, somewhere it says that Koharu is a high schooler. So she's like, around 15 at least? Akari is older than Rinko by 3 years, so she is 32 in the future. 32 - 15 = 17 years, which is Akari's age before the time skip. So yeah, agreed--if Koharu is Akari's kid, then Akari sure knows how to hide a pregnancy.