The age at which teeth erupt can vary widely. The primary teeth begin to form at 7 weeks in utero with mineralization beginning around the 4th month of fetal development. The eruption of teeth usually occurs symmetrically in each arch, with mandibular teeth erupting before the same maxillary teeth. The sequence of eruption is more important than the timing which varies greatly in both primary and permanent teeth. Variations of 6 months on either side of the usual eruption date may be considered normal for a given child. The first teeth to erupt are the primary mandibular central incisors at the approximate age of 5-8 months, with the maxillary central incisors following a month or two later.
A simple way to remember approximate eruption times is the "7 + 4" guideline.
At approximately 7 months, the first primary tooth erupts.
By 11 months "7 + 4" there are 4 erupted primary teeth.
At 15 months, 4 more primary teeth will have erupted for a total of 8 erupted primary teeth.
By the age of 19 months, the child should have an additional 4 erupted primary teeth for a total of 12 erupted primary teeth.
By the age of 23 months, 16 primary teeth should be present.
By the age of 27 months, 20 primary teeth should be present.