Long (and Wide) Division: How Many Weeks in a Pregnancy

An article on how many weeks elapse during a pregnancy, including the average timespan of pregnancy, how trimesters are divided and early or late pregnancy info. 

Determining how many weeks in a pregnancy is easy (38 to 42 weeks, on average) and hard at the same time, as no two women are alike and many pregnancies come early or late. In the United States, 8 to 10 percent of babies are born prematurely, or before the 37^thweek, while a small percentage continue past week 42. In these rare cases, pregnancy is induced for both the mother and the baby’s safety. Whether you have a caesarian or a natural birth, a single baby or triplets, the time spent in pregnancy will vary in ways that modern medicine has not caught up with. Betting some money in a delivery date pool is always a fun way to get your mind off it, though!

Linea nigra dark midline streak on a 22 weeks pregnant female. (Photo credit: Wikipedia)

The Trimester System

In obstetrics terms, pregnancy is divided into three trimesters that last 12 to 14 weeks each. Starting at the date of conception, which can be determined using a pregnancy calculator, the first trimester includes weeks 1 through the end of week 13, while the second trimester ends by the 26^th week and includes the fourth, fifth and sixth months. The final and third trimester includes the seventh, eighth and ninth months of the pregnancy, and usually ends between weeks 38 and 42.  These weeks refer to the weeks from the time of conception, and not the actual age of the fetus. The fetal age is approximately 2 weeks younger than the mother-to-be’s pregnancy (in weeks), because implantation, or the act of the fertilized egg entering the uterine wall, occurs 2 weeks after conception.

Weekly Landmarks

By week 13 (a fetal age of 11 weeks), the fetus will weigh approximately 1 ounce with an average height of three inches. By this stage, the external genitalia are formed, and your doctor may be able to tell the sex, although this is not guaranteed. At 13 weeks, you will not be able to feel much, but by the end of the 26^th week and the second trimester, you will feel (and show) quite a bit. At 26 weeks the fetus will weigh 2 pounds with a height of about 14.5 inches. Other landmark developments by this stage are the fetus’ hearing and eyesight. By the end of the third trimester, at the end of week 38, the baby is usually ready for delivery, but this may take up to1n extra month, or until week 42, at the latest. At this stage, the fetus weighs an average of 7.5 pounds and is 20 inches tall.

Premies and Latecomers

If born before the 37^th week of pregnancy, a baby is premature, or a premie. Premature babies require more specialized attention because they are often underdeveloped. The earlier a premature baby comes, the lower the chance for survival, although modern medicine has come a long way in this area. When a baby is a latecomer, or has not been delivered by week 42, labor is induced.

How many weeks in a pregnancy is an important question to ask at the start of the pregnancy, but the actual number of weeks will differ. As you visit your OB/GYN for regular checkups, she will be able to gauge the fetal growth and keep you on the right track.