A distinction is made between the biological order (or total order) and the order in the marriage. The biological or total order is the order of birth of the children in a woman's lifetime, irrespective of her matrimonial situation. The eldest is therefore the child numbered one in the order.
The order in the marriage concerns only the children of married women. This is the order of birth of the children born during the current marriage of the woman. It is always less than or equal to the total order.
For example, a woman has had three children. Barnabé was born when she was single. She subsequently got married. Emile was born. She then divorced and remarried. Jeanne was born during her second marriage. Barnabé is one in the total order, Emile is two in the total order and Jeanne is three in the total order. Barnabé has no order in the marriage, Emile is one in the marriage (the first) and Jeanne is also one in the marriage (second).
From 1998, files of the Civil status registry supply for all the children their birth order among the alive born children of the mother.