then; if so; if that is the case
Used to draw a logical conclusion or consequence from a preceding statement, similar to 'if that is the case, then...'. Often interchangeable with とすれば.
See also: とすれば
彼が来ないとすると、誰が代わりにやるの？
If he doesn't come, then who will do it instead?
それが本当だとすると、大変なことだ。
If that is true, then it's a serious matter.
If he's telling the truth, then the culprit must be someone else.
ならば is a more direct, general-purpose 'if so/if' used for hypotheticals or suggestions, while とすると specifically draws a logical consequence from a stated premise.
Derived from the conditional particle と combined with する (to do) in its -to form, literally 'if one does that, then...'. The exact historical development is straightforward from these components.