Pregnancy Basics

Pregnancy is usually dated from the first day of the last menstrual period, creating an expected term of about 40 weeks or 280 days. Gestational age is not just a calendar detail: it determines when screening, immunization, prophylaxis, fetal surveillance, and delivery decisions are appropriate. When menstrual dating is uncertain or conflicts with ultrasound, first-trimester crown–rump length provides the most reliable estimate. Pregnancy also changes the baseline against which findings are interpreted. Plasma volume, cardiac output, glomerular filtration, and insulin resistance rise; blood pressure may fall temporarily; and progesterone-related smooth-muscle relaxation can cause reflux, constipation, and urinary stasis. These expected changes must be distinguished from new hypertension, infection, bleeding, impaired placental function, or reduced fetal movement. Canadian care treats pregnancy as a normal physiologic process while adapting assessment and referral to individual risk, local resources, and the person’s preferences. Access and care pathways vary across provinces and territories, particularly for Indigenous, rural, and remote communities.