Final answer:
Infants should typically be able to hold their head up by the end of the first two months, with the skill improving by four months. The average milestone for head control is around 6 weeks old, with an expected range between 3 weeks and 4 months.
Step-by-step explanation:
A client, typically an infant, should be able to hold their head up and keep it steady while held in an upright position by the end of the first two months after birth. By four months, an infant not only improves this ability but also can lift their head 90 degrees while lying prone. It's important to note that motor skill milestones like these have an average age of achievement, but also a range. For head control, most babies can hold up their head at about 6 weeks old, and 90% achieve this between 3 weeks and 4 months old. Therefore, a client is generally expected to hold their head up by around two to four months of age. Infants displaying a delay in reaching this milestone by 4 months may require further evaluation by a pediatrician.