Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with your doctor.
Without the ability to examine your baby and obtain a history, I can not provide you with a specific diagnosis nor definitively say whether or not he has a problem with his/her neck muscles. However, I will try to provide you with some useful information.
The general term used for babies who can not hold their head up well is head lag. At birth, all babies have head lag. Some control over head movement is reached by 8 weeks so that head lag lessens. By the age of 4-6 months, most babies should be able to hold their head up when pulled by their arms to a seated position. However, there is of course some variation between infants, and some may hold their head up earlier and some a little later.
Head lag may be due to hypotnia, or low muscle tone. It may also be due to muscle weakness. Delays in other motor mile stones, such as reaching out for objects held in front of the baby at around 2-4 months, shaking a rattle if placed in the babies hand at around 6 months, sitting up at around 6-9 months, may suggest that there is generalized hypotonia (hypotonia in other muscle groups besides the head). The causes of hypotonia and weakness are multiple. As you can see, there is a range for the age when these milestones are expected to be met and some babies naturally are more delayed than other.
I suggest you have your baby seen by his/her pediatrician soon. After he/she examines him/her, he/she will be able to determine whether or not your baby might have a motor problem and whether or not referral to a pediatric neurologist is warranted.
Thank you for using the forum I hope you find this information useful good luck.
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