The age of an unborn baby is known as the gestational age. This calculation is often difficult to make completely accurately, as the exact date and time when conception occurs is rarely known. Therefore, it is typically necessary to base the number, counted in weeks, on a definite date such as the first day of the mother's last menstrual period.
In most cases, calculation of the gestational age assumes that conception has occurred 14 days after the first day of the last period. Since conception normally can occur anywhere from 11 to 21 days later, the use of 14 days is only an average estimate, with the potential for several days error. The due date of the baby is then assumed to be 40 weeks from the first day of menstruation; however, only a small percentage of babies are actually born on their expected due date. Women who want to know the estimated gestational age of their fetus and the child's expected due date can check with their doctor for that information; there are also a variety of websites that provide tools to do the calculation for them.
There are certain ways to estimate the gestational age that can provide more accuracy than basing it off of the last menstrual period. Typically, women will have specific physical signs that they are ovulating, such as changes in cervical position and mucous; those who are aware of these changes and also know when they had sexual intercourse may have a much more specific knowledge of what day conception happened. Ultrasound, particularly early in the pregnancy, can reveal physical indicators of the fetus's age. In cases where fertility treatments such as in vitro fertilization are used, the date and even the time of conception are definitely known. Once the baby is born, doctors may be able to evaluate its age based on physical development.
Knowing the true gestational age of a baby becomes particularly important at the time of birth. An infant born prior to 37 weeks is considered premature; babies in this situation are often susceptible to a variety of medical issues, such as intraventricular hemorrhages or respiratory distress. Understanding where the newborn is in its development can help doctors understand what issues are most likely to occur, and to develop a treatment plan which will give the baby the best possible care. Observation of certain physical characteristics such as length, weight, and reflexes right after birth help doctors make this determination.