Sveta can't tell who's child she having or there's no DNA test at clinics. Not long ago she could tell exactly who's child she carrying to her friends, mom. She is not doing any part of therapist quest line. What gibs?
Sadly it seems there's quite a few cases what she doesn't recognize as potential risks. I think most contraception methods make her discard the possibility of pregnancy even when all of them have a chance to fail. Ideally she could consider anyone as the father if they had sex, but right now there isn't a separate list for the fathers to contest her claims so the list is a bit strict to take both sides into consideration.
On a side note, I think my current character is having brown twins and she's going to tell her brother that they are his...
Oh, and if you absolutely want to have the father on the list you can add him with the console commands.
$wombpotfath[0] in console should return 'unknown' if you got nobody in the list and $wombpotfath[1] should be empty. So you can add $wombpotfath[1]='theID of your desired dad' into the console to add a new father. Like $wombpotfath[1]='A34' should add Kolka as potential father. You can use the NPC editor in cheat menu to find the IDs you need. Also disclaimer: I'm just a random person suggesting console commands without expertise. This should work, but if your game breaks I'm afraid somebody might point out not to use console commands in the first place.
EDIT: Not to double post. Did something change in the way pregnancy records the expected father? Just a few versions ago I was able to get brother talk about the pregnancy but now as I look the conditions for it to happen it has
if arrpos('$ChildThFath','Kolka') >= 0
which, as far as I can tell will never result to anything but -1 as the $ChildThFath is in forms of ID rather than first names the arrpos can never locate it from that array. So my kid being fathered by 'A34' still returns -1 in that check even when that ID is Kolka.