Development of the fetus

Essential information to rememberEmbryo at 40 days (7 weeks after the first day of the last menstrual period).

Essential information to remember

Fetus at the end of the first trimester.

Your baby is constantly growing and must go through several stages before he’s ready to live outside the uterus. These stages, or key moments, are outlined below. Remember, the number of weeks associated with each stage (based on last menstrual period) is only an approximation and may differ from one woman to the next.

First trimester: from conception to 14 weeks

At 5 weeks—about 22 days after conception—the embryo’s heart begins to beat, although it cannot yet be heard during a medical exam.

At 6 weeks, the embryo is 5 millimetres in length.

At 7 weeks, the embryo’s head is much bigger than the rest of its body. Its arms begin to form as the elbows and hands appear. The fingers are still fused together, but the eyes are now quite visible.

At 10 weeks, the embryo already has a human appearance: its eyes, nose, and mouth are recognizable. Its eyelids are closed. The fingers have now separated and the toes are beginning to form. Your baby begins to move his limbs, but you won’t feel any movement yet. He has now progressed from the embryonic to the fetal stage: all the body parts are in place, but are not yet fully formed. They will continue to grow and develop throughout the pregnancy.

Between 10 and 14 weeks, the fetus gets bigger and the skeletal bones begin to form.

At 14 weeks, the fetus measures 8.5 cm.

Second trimester: 15 to 28 weeks

Essential information to remember

Fetus at the beginning of the second trimester.

Essential information to remember

Fetus at the end of the second trimester.

At 14 weeks, your baby’s genitals, while not yet fully formed, are developed enough to reveal whether it’s a boy or a girl. Usually between 16 and 18 weeks, he or she may let you in on the secret when an ultrasound is performed.

At 16 weeks, the baby’s head is still disproportionately large compared to the rest of his body, but his trunk, arms, and legs are beginning to lengthen.

Around 20 weeks, your uterus is level with your belly button. Your baby’s movements are now strong enough that you can feel them. Some women feel these movements a little earlier or a little later in their pregnancy. Your baby is coated in a whitish cream known as vernix caseosa, which protects his skin.

At 22 weeks, your baby measures 19 cm. His hair begins to grow, and his body is covered in a fine downy fuzz known as lanugo.

Between 23 and 27 weeks, your baby puts on weight and his head becomes better proportioned to his body.

At 24 weeks, he can hear low frequency sounds from outside the uterus.

Around 26 weeks, his eyebrows and eyelashes are visible.

Around 28 weeks, your baby’s eyes begin to open. They will become sensitive to light at around 32 weeks.

Third trimester: from 29 weeks to birth

At 30 weeks, your baby measures 28 cm.

Essential information to remember

Fetus at the end of the third trimester.

At 32 weeks, your uterus is level with the upper part of your belly, known as the epigastric fossa. Your intestines, liver, and lungs are pushed upwards. As the pregnancy advances and the baby gains weight, the uterus expands outwards, stretching the abdominal muscles and skin.

At 36 weeks, your baby’s skin is pinkish, and the downy hair on his body begins to disappear, although it can remain until after the birth. Your baby is bigger because of the fat reserves he is building up.

Between 38 and 41 weeks, the baby has good muscle tone and may be active for longer periods at a time. He’s ready for the big day!

Photos: Lennart Nilsson