women s health issues
Detecting and Dating a Pregnancy
Due date
Pregnancies are conventionally dated in weeks, starting from the first day of the last menstrual period. After pregnancy is confirmed, the woman’s doctor asks her when her last menstrual period was. The doctor calculates the approximate date of delivery by counting back 3 calendar months from the first day of the last menstrual period and adding 1 year and 7 days. For example, if the last menstrual period was January 1, the doctor counts back 3 months to October 1, then adds 1 year and 7 days. The calculated due date is October 8 the next year. Only 10% or fewer of pregnant women give birth on the calculated date, but 50% give birth within 1 week, and almost 90% give birth within 2 weeks (before or after the date). Delivery between 2 weeks before and 2 weeks after the calculated date is considered normal. The ovaries usually release an egg (called ovulation) about 2 weeks after a woman’s menstrual period starts, and Fertilization usually occurs shortly after ovulation. Consequently, the embryo is about 2 weeks younger than the number of weeks traditionally assigned to the pregnancy. In other words, a woman who is 4 weeks pregnant is carrying a 2-week-old embryo. If a woman’s periods are irregular, the actual difference may be more or less than 2 weeks. Pregnancy lasts an average of 266 days (38 weeks) from the date of fertilization (conception) or 280 days (40 weeks) from the first day of the last menstrual period if the woman has regular 28-day periods. Pregnancy is divided into three 3-month periods, based on the date of the last menstrual period: 1st trimester: 0 to 12 weeks of pregnancy 2nd trimester: 13 to 24 weeks 3rd trimester: 25 weeks to delivery The most accurate way to determine when a baby is due is Ultrasonography , particularly if it is done during the first 12 weeks. 