DETROIT (AP) _ The legal definition of motherhood will be the issue when a judge seeks to untangle the parentage of an embryo that began its journey toward life in a lab dish and continued it in the womb of a surrogate mother.

Noel P. Keane, a Dearborn attorney, said Thursday that he will seek the precedent-setting custody ruling at a March 14 hearing in Wayne County Circuit Court. The motion is uncontested.

''Up to now, the courts have not defined motherhood,'' he said. ''We have always assumed that the woman who gives birth is the mother. In this particular case, the mother has no legal right to the child.''

The genetic mother of the child is a 37-year-old woman whose uterus was removed during a hysterectomy.

An egg was taken from her last summer and fertilized with her husband's sperm in a laboratory dish, a procedure called ''in vitro vertilization.''

The fertilized embryo was then implanted in the uterus of a 22-year-old Wayne County woman by Dr. Wulf H. Utian, co-director of the Laboratory for In Vitro Fertilization and Embryo Transfer at Mt. Sinai Medical Center in Cleveland. That woman already had two children.

In a letter to the New England Journal of Medicine that appeared in November, Utian said the pregnancy developed wihout problems. The woman is expected to give birth in April.

''This is, to my knowledge, the first case (involving a surrogate mother) in the world,'' he said.

Keane said Thursday that the surrogate mother, a friend of the couple involved, will receive a fee of $5,000 to $10,000 for bearing the child. He declined to identify the parents and the surrogate mother.

''This is the first time that a birth mother will not be placed on the birth certificate,'' he said.

The legal ruling would set a precedent because no state or federal laws govern custody after such a birth, Keane said.