When Is the Baby Due?

Posted on May 31st, 2008 by olga

Some statistics:
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 habitual 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

Usually pregnancies are dated in weeks, beginning from the first day of the last menstrual period.
Following pregnancy is proved; the woman’s physician asks her when her last menstrual period was. The doctor analyzes 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 instance, if the last menstrual phase was January 1, the doctor counts back 3 months to October 1, then adds 1 year and 7 days. The considered due date is October 8 the next year. Just 10% or fewer of pregnant women give birth on the calculated date, however 50% give birth within 1 week and almost 90% give birth within 2 weeks (earlier than or after the date). Delivery between 3 weeks ahead of and 2 weeks after the calculated date is regarded as standard.

Ovulation typically happens about 2 weeks following a woman’s menstrual period starts, and fertilization habitually takes place soon after ovulation. Therefore, the embryo is about 2 weeks younger than the number of weeks usually 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 unbalanced, the actual variation may be more or less than 2 weeks.

The most exact way to find out when a baby is due is ultrasonography, principally if it is done for the duration of the first 12 weeks.