The most accurate way to measure an infant's height is to let the infant lay flat on a table. One of the ways to make sure that the measurements are accurate is that the feet should be flat against a board and the knees should touch the table. The head should also lay flat on the table. These rules should also apply to the shoulders and the arms of the infant. Also, it would be better to take several measurements (or a few trials) just to make sure that your data is accurate and precise. Finally, an assistant could help you make sure that the "do's" mentioned above are executed.