Usually this can happen due to prominence of the anterior fontanelle (‘soft spot’) while crying or straining and is commonly seen in otherwise healthy babies. Since your child is active and achieving milestones, it is less likely to be serious. The soft spot normally gradually closes with age.
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However, if the bulge remains even when the child is calm, becomes tense, or if there is vomiting, fever, excessive sleepiness, seizures, poor feeding, or delayed milestones, then pediatric examination is advised.