Ermentrude was the daughter of unknown parents. The date and place of her birth have not been found.
She had two marriages/partners. Her first husband was Giselbrecht I van Bourghelles. They were married, but the date and place have not been found. Their only known child was Marie (?->1220).
Her second husband was Jean I. They were married, but the date and place have not been found. Their only known child was Élisabeth (?-?).