In the delivery room, when two babies arrive just minutes apart, the midwife records the time of each birth on two separate forms. It is this administrative detail, sometimes separated by just one minute, that determines for the civil registry which of the twins will be considered the elder. The rule seems simple, but it raises questions that parents of twins know well.
Declaration of birth of twins: what French law says
In France, the first twin born is declared the elder on the birth certificate. The civil registrar records each child separately, in the chronological order of their arrival in the world. Article 56 of the Civil Code requires a declaration within five days following the birth, and it is the time recorded by the medical team that is authoritative.
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In practice, each certificate shows a precise time. If the first baby is born at 2:02 PM and the second at 2:09 PM, the first will be listed first in the family record book. This chronology does not change, even in the case of a cesarean section where the interval can sometimes be less than a minute.
This classification does not have any direct legal consequences today. The right of primogeniture has been abolished, and inheritance rules treat twins like any other siblings. There is also a useful reminder of this legal framework applied to multiple births in the articles on Un P’tit Air de Famille.
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First born and first conceived: why medicine does not decide
A popular saying claims that the last born is the first conceived, as it would be “at the back” of the uterus. This idea circulates widely, but no medical data allows for determining an order of conception among twins.
For monozygotic twins (from a single fertilized egg that divides), the question simply does not make sense. The two embryos come from the same initial cell. There is no “first” or “second” at the moment of division.
In the case of dizygotic twins (two separately fertilized eggs), fertilization can theoretically occur a few hours apart. Early ultrasounds and non-invasive prenatal tests, available from the tenth week of amenorrhea, allow for monitoring the growth and position of each fetus. However, they do not provide any information about the order in which fertilization occurred.
Labeling Twin A and Twin B in ultrasound
During the monitoring of a twin pregnancy, the medical team assigns a label (Twin A, Twin B) to differentiate the fetuses in the reports. This designation is based on their position in the uterus, not on any order of conception. Twin A generally refers to the fetus closest to the cervix, the one most likely to be born first during a vaginal delivery.
Specialized centers like the Sunnybrook Twins Research Centre in Canada systematically use these labels in their databases to track the health trajectories of twins after birth. The order of birth then becomes a research variable, but it remains a practical reference, not an indicator of biological seniority.
Twin delivery and birth order: situations that complicate matters
The mode of delivery directly influences the interval between the two births, and sometimes even the perception parents have of seniority.
- During a vaginal delivery, the time between the first and second baby can vary from a few minutes to sometimes over half an hour. Twin A, positioned lower, generally comes out first. The ultrasound order and the birth order coincide in most cases.
- In the case of a scheduled cesarean section, the interval often falls under a minute. The surgeon extracts the babies quickly, and the order may differ from that observed on the ultrasound, depending on surgical access and the position of the fetuses at the time of the procedure.
- In some rare situations (presentation of the second twin in breech, fetal distress), an emergency cesarean on the second baby can extend the delay. Experiences vary on this point, as each twin delivery follows its own dynamics.
Psychology of twins and the place of the elder in the sibling group
Professor René Zazzo, a French authority on twinning, considered the question of seniority among twins to be arbitrary. However, his work shows that twins themselves often claim this order, even when the gap is just a few minutes.
In many families, the firstborn spontaneously takes on a “big” role, sometimes encouraged by parents who seek a differentiation reference. The book “The Guide to Twins” by Jean-Claude Pons, Christiane Charlemaine, and Emile Papiernik emphasizes that this attribution, although legally ineffective, structures the dynamic of the twin pair from early childhood.
Should we refer to the elder with twins?
Some parents choose not to mention the order of birth to avoid creating an artificial hierarchy. Others use it as a simple fact, just like birth weight or arrival time. No approach is universally recommended by early childhood specialists.
What emerges from family testimonies is that twins almost always end up discovering their birth order, if only by consulting their family record book. It is better to discuss it simply than to make it an unnecessary secret.
Seniority among twins remains an administrative reference, not a biological truth. The midwife records a time, the civil registrar records an order, and the family takes it up (or not) as they see fit. French law gives no advantage to the firstborn, and medicine confirms that we will probably never know which was conceived first.