Recently saw a video by a nurse swinging a baby probably under 1 year holding baby's legs.
Is it safe to lift 7-month-old girl baby using the legs if the baby enjoys?
This depends somewhat on the baby's age and whether the baby has any problems with his hips. If a baby is cruising (standing holding on to furniture and taking steps this way), I don't think short periods of gentle upside down swinging is harmful; obviously, don't repeat if the baby isn't enjoying it. I would also add that it shouldn't be done if a baby has recently eaten.*
The hips are comparatively very stable joints; it's rare to dislocate a normal hip, whereas dislocated shoulders are common, and dislocated elbows are very, very common. That's because one of the pair of bones in the forearm, the radius, is incompletely formed still, and the ligaments still relatively weak, and do not protect from being pulled out of joint with force.
Nursemaid's elbow often occurs when a caregiver holds a child's hand or wrist and pulls suddenly on the arm to avoid a dangerous situation or to help the child onto a step or curb. The injury may also occur during play when an older friend or family member swings a child around holding just the arms or hands.
Nursemaid's elbow occurs when there is a partial separation of the radiocapitellar joint. Because a young child's ligaments - the strong tissues that attach bones to each other-are not fully formed, even a mild force on the joint may cause it to shift, or partially dislocate.
The annular ligament surrounds the radius and may be particularly loose in some young children, which may lead to nursemaid's elbow recurring over and over again.
Although the source mentions swinging a child by the arms, I have never seen a dislocation of this kind occur if this is done gently and properly (both hands - or wrists - at the same time.) Otherwise, with the popular "swing me" game (where the child is walking between the parents and asks to be swung) would result in many injuries.
Of course, this should be avoided in children who have already experiences a dislocated elbow (or more properly called a radial head subluxation, which is a partial dislocation) because the first recurrence rate is about 22%.
*My husband was doing this to our (maybe 6 month old) baby, who was loving it. I asked him to stop, I encouraged him to stop, I warned him to stop, but he didn't, until the child vomited in a large arc everywhere. He should have thought about it more. Common sense. But the baby was fine, if perhaps a little dizzy on returning to the proper upright position.
The hips are so stable normally that on delivery of sheep and goats, if the newborn animal is not breathing, one of the first things the 'midwife' (me, in my case) can do is to grab the newborn by the hind legs and forcefully swing in a full circle. This helps to clear liquid from the respiratory tract. NB: sheep and goat infants's hips are strong enough to withstand walking/weightbearing a very short time after birth.
The biggest risk I see is hitting the head on something (if swinging) or dropping the baby (head/neck injury). You might think you would never do that... but accidents do happen. Imagine you're holding her close to your face and you suddenly get a stream of vomit coming your way. It could be reflex to put her down a bit more quickly than you should.
So at that age I would only be comfortable doing it a very short distance above a soft surface, with slow movements and no hard objects nearby.