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A month-by-month guide to fetal growth

When you’re pregnant, it’s only natural to wonder what the baby growing inside you looks like—and how it’s changing and developing as time passes. In words and pictures, this article will give you a sense of the amazing monthly progression.

First Month: Four weeks into pregnancy, the embryo is about half an inch long and weighs less than an ounce. The heart is beating. Arm and leg buds, the forerunners of limbs, begin to appear. This is a critical time for development, and the embryo is especially sensitive to drugs or infections that can cross the placental barrier.

Second Month: By the end of the second month, the embryo still weighs less than an ounce. But all the major body organs, such as the heart, brain and lungs, are growing. Ears, ankles, wrists, fingers and toes are taking shape. The first bone cells appear. The head takes up one-third of the embryo’s body size. The embryo begins to move around in the sea of amniotic fluid that cushions it from bumps and blows.

Third Month: The baby is now called a fetus rather than an embryo. It will grow to 4 inches long and weigh just over 1 ounce by the end of this month. Soft nails are growing on fingers and toes, and tooth buds appear. Hair begins to grow on the fetus’ head. Reproductive organs are developed, but it’s still too early to tell the baby’s sex. The first three months are the most critical time for organ growth and development. From now on, the threat of miscarriage lessens.

Fourth Month: Now 6 to 7 inches long and weighing about 5 ounces, the fetus can suck, swallow, kick, hear and urinate. The fetus’s head begins to look more appropriately proportioned as the body begins to grow faster than the head. The facial features continue to develop in a complicated set of processes that will eventually allow the fetus to frown, cry and laugh.

Fifth Month: Finally, Mom can feel the fluttery kicks and pushes of the fetus, now measuring 8 to 12 inches and weighing 1/2 to 1 pound. The fetus settles into a regular pattern of sleep and wakefulness. Soft, thick, downy hair called lanugo covers the fetus, which now has eyebrows and eyelashes. Vernix, a waxy substance that protects the fetus’s skin from the constant exposure to amniotic fluid, coats the body.

Sixth Month: At the end of the sixth month, the fetus will be 11 to 14 inches long and weigh 1 to 1 1/2 pounds. A baby born now—still very premature—often can survive in intensive care. The fetus is covered by a thin skin, beneath which there’s no fat. Delicate finger and toe prints have developed, and the fetus’ eyelids can open and shut.

Seventh Month: The fetus weighs about 3 pounds and is about 15 inches long. Its bones are hardening, except those of the head. These remain soft and flexible for delivery, when the head must mold to the contours of the birth canal. From now on, the fetus will begin to stockpile body fat and weight in preparation for its birth, when some weight loss is common. It can respond to light and sound. (When born it will recognize Mom or Dad’s voice!) If the baby were born now, still premature, survival chances would be good.

Eighth Month: By the end of this month, the baby will be about 18 inches long and weigh about 5 pounds. Although it’s too big to move around much, its kicks have become stronger. The baby’s brain is growing rapidly. Its lungs may still be immature, but if born now, survival chances would be extremely good.

Ninth Month: The baby gains about half a pound each week during this last month. You might not feel the baby move as much because there’s little room to spare. Most likely the baby will settle into the birth position: head first with its knees tucked up near its nose. It may have dropped down into the pelvic region. At birth, babies average 20 inches long and 7 1/2 pounds.