Gestational age relates to the age of an embryo or fetus (or newborn infant). There is some ambiguity in how it is defined:
In embryology, the term "gestational age" is seldom used because it lacks precision. The timing of embryonic development starts with fertilization. In mammals, this proceeds through uterine implantation, until birth.
In human obstetrics, gestational age is often defined as the time elapsed since 14 days prior to fertilization. This is approximately the duration since the woman's last menstrual period (LMP) began. There is also a further distinction between the calendar gestational age described here, and the developmental gestational age determined by comparing an embryo or fetus to the average age of others that were at the same stage of development.
Unless the exact date of fertilization is known, counting from LMP has been the common method of computing gestational age. It involves the assumption that fertilization in humans typically occurs a consistent period (14 days) from the onset of the LMP. Although this "LMP method" of calculating gestational age is convenient, other methods are in use or have been proposed.
- Large for gestational age: Weight is above the 90th percentile at gestational age
- Macrosomia: Weight is above a defined limit at any gestational age
- Appropriate for gestational age: Normal birth weight
- Small for gestational age: Weight is below the 10th percentile at gestational age
- Low birth weight: Weight is below a defined limit at any gestational age