I think the standard fifteen seconds is adequate, rather than a full minute. The foetal heart generally remains normal throughout first stage. But in assessing this, it is important to know what the usual reading was throughout pregnancy, and also allow for the time of day. Problems can arise if you do not know what variations to expect in second stage. For most of second stage the foetal heart usually remains normal, but some babies' hearts can become erratic in second stage, without indicating any serious problem.
Some babies are sensitive and excitable.
Moulding of the skull can be a stimulus. When the baby's head is on view, you should not try to check the heart, because it sometimes happens that the oxygen is cut off when it has descended to the perineum, owing to shrinkage of the uterus over the placental site, cutting off maternal supply to the placenta. When this happens, the baby's heart slows drastically, and if you don't expect it, you could think that the baby was in trouble, when actually it isn't, because the slowing of the heart is a reflex action to conserve oxygen, but baby is quite safe because the normal polycythaemia in term babies ensures very adequate reserves of oxygen. Even when baby becomes blue, it is not in danger, it still has reserves. A shocked baby is not blue, but pale grey.