The law has foreseen this issue by specifying in Articles 88 and 89 of the Law on Marriage and Family 2014 as follows:
Article 88. Identification of parents
1. A child who is born or conceived by the wife during the marriage period is the common child of the husband and wife.
A child who is born within 300 days from the time of termination of a marriage shall be regarded as a child conceived by the wife during the marriage period.
A child who is born before the date of marriage registration and recognized by his/her parents is the common child of the husband and wife.
2. When a parent does not recognize a child, he/she must have evidence and such non- recognition shall be determined by a court.
Article 89. Identification of children
1. A person who is not recognized as the parent of a person may request a court to identify that the latter is his/her child.
2. A person who is recognized as the parent of a person may request a court to identify that the latter is not his/her child.
Typically in Judgment 107/2018/HNGĐ-ST dated November 29, 2018 on determining the father for a child. According to the judgment, Mr. Nguyen Minh S and Ms. Vo Thi Q (not divorced from Mr. Tran Van N) live together as husband and wife. On September 27, 2013, Mrs. Vo Thi Q gave birth to Tran Thi Kim Mai during her marriage to Mr. Tran Van N, so in Mai's birth certificate, she has a mother named Vo Thi Q and a father named Tran Van N.
Mr. Nguyen Minh S suspected that Mai was his child with Mrs. Q, so he took a DNA test and according to the DNA test results of the Scientific Council of the LOCI Institute of Molecular Biology, the person with the DNA sample with the symbol Nguyen Minh S and the person with the DNA sample Nguyen Thi Kim Mai has a father-son blood relationship, with a frequency of 99.99999789%. Therefore, Mr. S asked the Court to identify Mai as the biological child of Mr. and Mrs. Q.
The court found that although Tran Thi Kim Mai was born during the marriage between Mrs. Vo Thi Q and Mr. Tran Van N, according to the DNA test results, Mai was Mr. S's daughter. On the other hand, Mr. N also determined that Mai was not his common child with Mrs. Q. Therefore, accepting Mr. S's request, he declared Mr. Nguyen Minh S to be the biological father of Tran Thi Kim Mai/
In conclusion, if you have a child with a married person and want to identify the child as your child, you can file a lawsuit to ask the court to identify the child in accordance with the law of Vietnam