It is fascinating to see babies develope and learn new skills.  Every baby is different and so is the development. Some babies do things sooner than the others. Sometimes they may be having movements which might happen suddenly and repeatedly. These can be normal or abnormal. Some babies are very active and can move their arms and legs, try to lift their trunk and these may appear abnormal. Usually these are normal and you can stop them if you hold them.

Abnormal movements are usually sudden and happen repeatedly. It is difficult to stop them by holding the baby as they are due to abnormal electrical activity in the brain. The most common and serious ones are ‘infantile spasms’, in which the baby suddenly startles and this keeps repeating for few seconds. The baby may be staring and vacant when these are happening. The diagnosis is confirmed by an EEG (trace of the brain activity). If you are concerned about your baby movements, you should see your GP or specialist.

https://www.epilepsy.org.uk/info/syndromes/west-syndrome-infantile-spasms