A digital thermometer with a flexible tip is the most practical choice for home use. Underarm readings suit newborns; rectal readings remain the most accurate for infants under three months.
Always take the temperature the same way, at rest — not right after a bath or a feed. Note the reading, the time, and how your baby behaves: pharmacists and paediatricians need all three.
Keep one thermometer in the nursery and a spare in the changing bag, so you are never caught out. Our twin pack was made exactly for that.