But that is not always the case, for many reasons, and as this new mom wrote to explain to New24, her situation has her wondering if she did the right thing.
Read her story here, then find out what the professionals advise in this tough situation.
Conceal this secret for a lifetime?
"Please, I need advise. I was so confused when I found out I was pregnant and who the real father is. So I told them both!
My boyfriend is so responsible and never doubts me. The other one has his own family and really wants to protect his reputation, but took the responsibility too and is secretly giving financial support to me.
Months passed, the situation was consistent. Even without DNA testing, I'm sure that my boyfriend is the real father of my child.
My question is, should I tell the other man that I was mistaken and apologise that he is not the father? Or tell him I choose to stay away from him and never accept any support, since he advised me to marry someone else?
Should I continue my life with my boyfriend and our child, and conceal this secret for a lifetime?"
We asked Felicity Guest, founder of Child Maintenance Difficulties South Africa, for insight.
"I would suggest she and her boyfriend do a DNA test, then everybody is clear and it can't be used at a future date against either of them," she advises.
"We don't know what the future holds, should they break up any uncertainty about paternity can hold up a maintenance order as DNA will probably be asked for," she says, adding " and more importantly the child should know who their biological father is."
Guest adds that the father should be certain it is his child, and urges her to consider the psychological trauma of thinking it is his child, and it is not.
She warns also that whoever is not the father in this situation can legally claim back all the money that he has given while thinking the child was his.
Claiming back maintenance
"In the event that there is an maintenance order in place and paternity is disputed, the best cause of action for the father would be to make a civil claim against the mother," she advises.
He would then use the amount he was paying in terms of an order to quantify a civil claim. If there is no order it will be more difficult to quantify and the father may have to employ the services of someone to assist in quantifying how much they may have contributed over the past months, or years.
"It would obviously be helpful in the father’s claim if they have proof of certain contributions made," she says, adding that it is quite a difficult action to make.
Proof of paternity
"However the best thing any father can do in an instance where they are uncertain of paternity is to request a paternity test. Although it may cause conflict initially if the test is perhaps refused by the mother; the father has the last option of withholding maintenance whilst issue of paternity is in question," she explains.
The onus would then be on the mother to approach the maintenance court, in which instance the father can raise the question of paternity and the maintenance dispute would be placed on hold pending paternity results.
"If the mother again refuses the paternity test then the presumption is in favour of the father that the child is not theirs and the mother will be unable to claim maintenance," she says.
Ask us! Do you have a legal question you need help with? Share your questions via email at firstname.lastname@example.org and we may speak to a legal professional on your behalf. Anonymous contributions are welcome.
Don't miss a story!
For a weekly wrap of our latest parenting news and advice sign up to our free Friday Parent24 newsletter.