A baby with a small biparietal diameter of 3 weeks is not always in trouble. When pregnant women undergo ultrasound examination, sometimes the baby’s biparietal diameter will be found to be small. Most of the babies with 3-week small biparietal diameter are caused by malnutrition and slow development, and some babies with developmental abnormality may also result in small biparietal diameter. Sometimes the date of pregnancy may be inaccurate due to inaccurate menstruation, for example, the estimated date of pregnancy may be longer than the actual date of pregnancy, and the result of the test may indicate that the baby’s biparietal diameter is 3 weeks small, but in fact, the baby’s biparietal diameter and the month of pregnancy are the same. Individual babies with oval-shaped heads that are slightly longer from front to back may also have a small biparietal diameter. It is recommended to have regular checkups during pregnancy and follow up with further review, prenatal diagnosis, etc. as prescribed by the doctor in case of special conditions.