It is normal for a 45-day-old baby to spit up because the stomach is horizontal and the cardia in the middle of the esophagus and stomach is loose, so it is easy for the milk to flow backwards and thus spit up. If your baby is in good spirits, eats milk normally and urinates normally after vomiting, you can take good care of him/her, avoid choking and coughing, and burp him/her in time, as he/she grows older, he/she will be relieved gradually. If your child spits up more milk, you can elevate the upper body properly. If your baby spits up more intensely and affects the growth and development of your child, and if there is no weight gain and urine output decreases, you need to be alert to whether your baby has congenital hypertrophic pyloric stenosis, and you need to go to the hospital in time to improve the relevant examination.