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The basic outlines of the organ, body, and nervous systems are established.
By the end of the embryonic stage, the beginnings of features such as fingers, eyes, mouth, and ears become visible.
Twins and other multiple births are counted as one pregnancy and birth.
A woman who has never been pregnant is referred to as a nulligravida.
Fertilization usually occurs about two weeks before the next expected menstrual period.
The chronology of pregnancy is, unless otherwise specified, generally given as gestational age, where the starting point is the woman's last normal menstrual period (LMP), or the corresponding age of the gestation as estimated by a more accurate method if available.
Sometimes, timing may also use the fertilization age which is the age of the embryo.
Naegele's rule is a standard way of calculating the due date for a pregnancy when assuming a gestational age of 280 days at childbirth.
Furthermore, actual childbirth has only a certain probability of occurring within the limits of the estimated due date.
A study of singleton live births came to the result that childbirth has a standard deviation of 14 days when gestational age is estimated by first trimester ultrasound, and 16 days when estimated directly by last menstrual period.