Difficulty to maintain correct position of the head while standing or sitting. Infant head lag is observed when the head seems to flop around or lags posteriorly behind the trunk. Several articles have maintained that head lag should be absent by age 3 to 4 months. [ https://orcid.org/0000-0002-0736-9199 http://www.ncbi.nlm.nih.gov/pubmed/33620859 ]