Twin A is most often born first.
When twins are identified during pregnancy, they are typically labeled as Twin A and Twin B. This antenatal labeling is crucial for monitoring their development and planning their delivery.
Understanding Twin Labeling and Birth Order
The assignment of "A" and "B" to twins before birth is not arbitrary; it follows a specific convention based on their position within the mother's uterus relative to the birth canal.
How Twins are Labeled Antenatally
- Location-Based: During prenatal ultrasound examinations, twins are labeled A or B according to their location. Twin A is the fetus positioned closest to the birth canal. Twin B is the fetus located further away.
- Early Establishment: This labeling is usually established early in the pregnancy by prenatal ultrasound and serves as a consistent identifier throughout the gestational period.
Correlation with Birth Order
While Twin A is identified by its proximity to the birth canal, there's a strong correlation between this antenatal label and the actual birth order.
- Common Outcome: In the vast majority of cases, the fetus identified in utero as Twin A is indeed the first one to be born. Consequently, they retain their "A" label even after birth. Twin B typically follows as the second born.
- Consistency: This consistent labeling helps medical professionals track each twin's growth and health throughout pregnancy and delivery.
The following table summarizes the common understanding of twin labeling and birth order:
| Twin Label | Antenatal Assignment Basis | Typical Birth Order |
|---|---|---|
| Twin A | Located closest to the mother's birth canal | Usually first born |
| Twin B | Located further from the mother's birth canal | Usually second born |
For further details on twin labeling and related topics, medical resources such as those found in journals like the American Journal of Obstetrics & Gynecology provide comprehensive information.
