The total lung capacity decreases because of encroachment of the lung space by intra-abdominal organs as the growing fetus pushes them into the chest cavity. To compensate, the size of each breath is larger. The mother's respiratory reserve capacity drops. Gastric motility and emptying is decreased which can exacerbate reflux. The normal pregnancy duration is 37-42 weeks.
Pregnancy duration is measured from the first day of the last normal menstrual cycle.
A simple rule to estimate the due date is take the first day of the last menstrual period and subtract 3 months, then add one week. Patients may be unsure of their menstrual period dates and it is common to estimate due dates by ultrasound exam done by the patient's obstetrician. The description of the patient's obstetric history is straightforward. Gravida is a term used to describe the total number of times a woman has been pregnant regardless of the outcome. Parity describes the total number of pregnancies that have been carried to a length generally considered to be viable.