All characters with a vagina can go into heat periodically or by taking heat pills which you can get from looting enemies/specific locations or buying them at the vending machine in medical. This basically guarantees impregnation but you can increase your characters "fertility(stat?)" by taking the skills in the fertility skill tab which you get XP for by giving birth.
p.s. you can take infinite showers (~30mins in game) at late night so if you want to run down the clock on your current pregnancy before the next day.