During the transition from primary to permanent dentition, a maxillary midline diastema is most likely to close spontaneously with continued eruption when its width is:

During the 'ugly duckling stage,' a maxillary midline diastema commonly narrows and closes spontaneously as the permanent canines erupt and migrate mesially — this self-correction is most reliable for gaps smaller than about 2 mm; diastemas wider than 2 mm are less likely to close fully on their own and more often warrant orthodontic evaluation.