Baby Development Guide for 0-40 Weeks During Pregnancy

In 40 short weeks, your baby grows from a small group of cells to a full-term infant. Here is a guide to your baby's development in the womb.

  1. Trimesters of Pregnancy

    • Pregnancy is divided into three trimesters: the first trimester lasts from day one to week 12, the second trimester consists of weeks 13 through 26 and the third trimester lasts from week 27 until the baby is born, generally before week 42. Weeks are calculated from the first day of the woman's last menstrual cycle. A baby is considered full-term if born after week 37.

    Fetal Terms

    • Your baby starts as a zygote--a fertilized egg cell that contains all the DNA needed to grow into an individual human. The zygote then divides to create a group of inner cells with a harder outer shell. This is called a blastocyst. About day six of pregnancy, the blastocyst implants into the uterus wall. The cells then divide and specialize in their functions. At this point, your baby is called an embryo. At the end of week eight, your baby is called a fetus.

    Baby Development in the Womb

    • A baby grows from a tiny zygote into a 6 to 12 lb. baby in only nine to 10 months. At week six, the embryo is about 1/5 of an inch long. By 10 weeks, the embryo is about one inch in length and still weighs less than 1 oz. By the 14th week, the embryo weighs about 1 oz. and is three inches long. The third trimester starts at 27 weeks; at this time, the fetus weighs up to 2 lb. and is about 14.5 inches long from head to toe. Babies typically weigh about 7 lb. by week 37, when the baby is considered full term.

    Fetal Features

    • During the first trimester, your baby is turning from a zygote into an embryo with noticeable features. The placenta forms at week four, and by week six the embryo has a primitive heartbeat, and its head, liver, mouth and intestines take shape. By week 10, facial features and limbs are distinguishable. Muscles and sex organs develop by the 14th week, or the beginning of the second trimester. At 18 weeks, the fetus can blink and grasp and hair starts to grow. Skin turns from transparent to opaque at 22 weeks. Eyes are completely formed at the start of the third trimester, or week 26.

    How Your Health Affects Your Baby

    • Excess stress can cause preterm labor, low birth weight and other birth complications. Your pregnant self directly affects your baby, so try practicing yoga or meditation to maintain a relaxed environment for your growing fetus. Do not smoke, drink alcohol or do drugs while you're pregnant.

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