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I am very sorry to hear your baby is in the hospital!

Could you please give me more details in order to help you? 36.1ºC is a normal temperature if taken in the axilla.

If the temperature was taken orally or rectal, than it is a little low, but this is not a reason for hospitalization if it is the only symptom

Hypothermia or low body temperature at this age can be due to a severe infection. Sometimes very young babies do not develop a fever, but get hypothermic.

Another reason could be an endocrine problem, such as hypothyroidism or adrenal insufficiency.

Also, a neurological problems localized in the brain centers responsible for thermal regulations can produce hypothermia.

All the causes I've mentioned should give other symptoms.

If she is free of any other symptoms, this is probably only related to the fact that her weight is a little low. Also, if she is premature, this can be due to it, since premature babies have difficulties in regulating their body temperature.

I see you are offline so I'll save this chat and switch to the Q&A mode. Feel free to post back if you have more doubts and please rate my answer only if you are fully satisfied with it. Otherwise, let me know how can I help!

I hope this is not too serious and she can go back home very soon!

Best wishes!