Undergo a paternity test at an accredited DNA testing facility. DNA paternity tests cost between $400 and $2,000 and require a DNA sample from your child. Although Missouri does not require the results of a paternity test to amend a birth certificate when no one disputes your paternity, the results of a paternity test will establish your paternity beyond reasonable doubt and could be useful in case a paternity dispute arises later.
Obtain a copy of your child's birth certificate. Since you have not yet established parternity, the child's mother must obtain this document for you. She can obtain a copy from any local health department in Missouri for a fee of $15 per copy by presenting a government-issued photo ID, filling out an application form and having it notarized.
Complete an Affidavit Acknowledging Paternity available on the Missouri Department of Health and Senior Services website. This form requires information that appears on the child's birth certificate, as well as information about the mother and father, such as places of birth, dates of birth and current addresses.
Execute the Affidavit Acknowledging Paternity by signing together with the child's mother in the presence of a notary public or two witnesses. If the notary public witnesses the signing, he must place his seal on the affidavit. If two witnesses are used, they must sign and date the affidavit and provide their addresses.
Mail the Affidavit Acknowledging Paternity to the Missouri Department of Health & Senior Services, Attn: Amendment Unit, P.O. Box 570, Jefferson City, MO 65102, together with a filing fee of $15 (the fee as of 2011). The child's birth certificate will be amended to include your name as the child's legal father.