Because a 45-day-old baby with constipation is too young, he should focus on whether he has congenital megacolon, intestinal malformation or abdominal problems, so he should promptly see a pediatric gastroenterology clinic, which may require abdominal color ultrasonography and, if necessary, a barium enema to rule out congenital megacolon, abdominal problems or intestinal malformation. If there is no problem, the main consideration is indigestion or food allergy. You can give your baby appropriate massage on the abdomen to promote intestinal peristalsis and relieve the symptoms of constipation. You can also give your baby a bifidobacteria-based probiotic to regulate the gastrointestinal function, and you can also take lactulose orally to laxative when there is difficulty in defecation. If the above treatment still cannot be relieved, you still need to go to the hospital for review.