Baby development guide for 0-40 weeks during pregnancy

Updated July 19, 2017

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.

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 fertilised 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 specialise in their functions. At this point, your baby is called an embryo. At the end of week eight, your baby is called a foetus.

Baby Development in the Womb

A baby grows from a tiny zygote into a 2.72 to 5.44kg. 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 28.4gr. By the 14th week, the embryo weighs about 28.4gr. and is three inches long. The third trimester starts at 27 weeks; at this time, the foetus weighs up to 0.907kg. and is about 14.5 inches long from head to toe. Babies typically weigh about 3.18kg. 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 foetus 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 labour, 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 foetus. Do not smoke, drink alcohol or do drugs while you're pregnant.

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About the Author

Sarah Valek has been a writer since 2006. She holds a Bachelor of Arts degree in writing and art from Ithaca College. Valek's work has appeared in, "Mommies Magazine" and "Natural Moms Talk Radio."