Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services if a newborn or infant:
Slow weight gain without urgent symptoms usually needs a routine appointment rather than emergency care.
During pregnancy, contact your maternity unit promptly if your baby is moving less than usual or there is a clear change in their normal movement pattern.
Do not wait until the next routine appointment.
Urgent assessment is also needed for vaginal bleeding, leaking fluid, regular painful contractions before term, severe headache, vision changes, sudden swelling, or severe pain beneath the ribs.
These symptoms do not necessarily mean the fetus is small, but they can occur with pregnancy complications that affect fetal wellbeing.
Seek same-day medical advice if the baby is feeding much less than usual, repeatedly vomiting, unusually sleepy, cold or feverish, increasingly jaundiced, or producing fewer wet diapers.
Follow any discharge instructions about blood sugar, feeding, and temperature.
Arrange a review if the child is not following their expected growth pattern, has persistent feeding problems, or is not meeting developmental milestones.
Every sign above is quoted from the sofidex article on Small for gestational age. If you are not sure, err upward — it is always reasonable to be checked.