Human Gestation Period: Week-by-Week Explanation of Fetal Development During Pregnancy

During pregnancy, the term "gestation" is frequently discussed. Continue reading as we delve into how gestation is connected to human pregnancy and explore related terms like gestational age.

What Is Gestation?

According to Healthline, gestation refers to the time between conception and birth and applies to all mammalian pregnancies, including humans. The gestation period in human pregnancies typically lasts between 38 and 42 weeks. Babies born before 37 weeks are considered premature, while those born after 42 weeks are termed postmature.

Since the exact date of conception is often unknown, gestational age is commonly used to determine the progress of a pregnancy. It is measured in weeks from the first day of the woman's last menstrual period, considering the period as a signal that the body is preparing for pregnancy, even though conception occurs later.

Fetal growth starts at conception when a sperm fertilizes an egg. Gestational age can be determined through ultrasound measurements during pregnancy, which assess the size of the baby's head and abdomen.

As a result, fetal age is approximately two weeks behind gestational age, making it less precise to measure pregnancy since the actual date of conception is often uncertain in humans.

Week-by-Week Fetal Development

Cleveland Clinic explains that the fetus and the expectant mother undergo distinct changes during each of the three trimesters, which are approximately three months long, with fetal development typically discussed in terms of weeks and are divided in trimesters.

First Trimester (Months 1-3)

• Weeks 1-2: The body prepares for a potential pregnancy by gradually releasing more hormones, and uterus gets ready for conception. At the end of the second week, ovulation occurs and pregnancy may begin if a sperm fertilizes the egg.
• Week 3: Fertilization takes place.
• Week 4: The fertilized egg becomes a blastocyct or small cluster of cells and implants into the uterine lining.
• Week 5: The central nervous system forms, and the tiny "heart" tube starts beating around 110 times a minute by the end of this week.
• Week 6: Buds that will become the arms and legs emerge, blood cells begin to take shape, and circulation starts. Structures for the ears, eyes, and mouth begin to take form, and a heartbeat can be detected through a vaginal ultrasound.
• Week 7: Bones start replacing soft cartilage, and the embryo's genitals begin to form. The head appears larger compared to the body, resembling a small tadpole or seahorse with a prominent tail and a large head.
• Week 8: Major organs and body systems continue to develop, and the fetus has web-like hands and feet. Eyes become visible, ears begin to form, and the umbilical cord is fully developed, facilitating the transport of oxygen and blood to the embryo.

• Week 9: Teeth and taste buds are beginning to form, and the fetus's muscles are developing. A Doppler ultrasound may detect its heartbeat.
• Week 10: The arms, hands, fingers, feet, and toes are fully formed, and the webbed fingers have disappeared. The external genitals are also beginning to take shape, but they may not be visible on an ultrasound yet.
• Week 11: The fetus is becoming more active, opening and closing its fists and mouth. Its knees, elbows, and ankles are functional, although it's too early to feel any kicks. Bones are solidifying, but the skin remains transparent. Facial features are becoming more distinct.
• Week 12: All major organs, limbs, bones, and muscles are now present and will continue to develop to become fully functional.

Second Trimester (Months 4-6)

• Week 13: The fetus's vocal cords are forming, and its head starts to grow in proportion to its body.
• Week 14: The fetus's skin thickens, and fine hair begins to appear. It gains the ability to bring its fingers to its mouth and turn its head. External genitals are fully developed, and fingerprints start to form.
• Week 15: Intestines and ears are relocating to their permanent positions. While still relying on amniotic fluid for breathing practice, its lungs are beginning to develop.
• Week 16: The fetus now has lips, and its ears are developed enough to hear sounds, including voice. Although its eyes remain closed, the fetus can react to light by turning away from it.
• Week 17: The fetus's skin is still thin, but it will start accumulating fat.
• Week 18: The fetus is now covered in lanugo, a soft hair resembling peach fuzz, providing warmth and added protection.
• Week 19: The fetus grows stronger, and most people start feeling its kicks and punches. It develops its own unique set of fingerprints and can experience hiccups.
• Week 20: Nails start growing on the tips of the fetus's fingers, and the part of the brain responsible for the five senses begins to develop.
• Week 21: The fetus now has bone marrow, aiding in the production of blood cells.
• Week 22: The fetus's grip strength is improving, and it can touch its ears and the umbilical cord. It becomes capable of hearing its mother's heartbeat, stomach sounds, and breathing.
• Week 23: If born prematurely, the fetus has a chance of survival with intensive care. It starts rapidly gaining fat.
• Week 24: While the fetus's lungs are fully developed, they are not yet mature enough to function outside the uterus.
• Week 25: The fetus's skin becomes plumper and less wrinkled due to increased body fat. Its nervous system undergoes rapid maturation.
• Week 26: The fetus starts producing melanin, responsible for skin and eye color. Its lungs begin producing surfactant, aiding in breathing after birth.
• Week 27: The fetus can now open its eyes, blink, and has developed eyelashes.
• Week 28: The fetus may begin to turn head-down in preparation for birth. By the end of the seventh month, it is about 14 to 15 inches long and weighs between 2 and 3 pounds.

Third Trimester (Months 7-9)

• Week 29: Mothers may now feel pokes from the kicks and jabs due to the limited space in the amniotic sac.
• Week 30: The fetus can now regulate its own body heat, and its brain is rapidly maturing and growing.
• Week 31: The fetus can process more information and stimuli as well as distinct patterns in its wakefulness and sleep.
• Week 32: The fetus's skin is no longer translucent, and apart from the lungs and brain, most other organs are well-formed and prepared for birth.
• Week 33: The fetus's bones are becoming stronger.
• Week 34: The protective vernix on the fetus's skin starts to thicken.
• Week 35: The fetus's brain continues to grow, but it still weighs only about two-thirds of what it should at birth.
• Week 36: The fetus sheds its lanugo, and hair starts to grow on its head.
• Week 37: The fetus's toenails have grown to the end of its toes, and you may feel it dropping into your pelvis.
• Week 38: The fetus is gaining around 0.5 pounds per week as it reaches its final size.
• Week 39: The fetus is now full-term and prepared for birth!
• Week 40: Due date week. Signs of labor may be felt.

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