From the moment of conception, a baby begins to develop in the womb, and over the course of nine months, undergoes an incredible transformation. Each week brings new changes and developments, as the baby grows and develops into a fully-formed human being. In this article, we’ll take a week-by-week look at how a baby develops in the womb, from the moment of conception to the day of delivery.
Table of Contents
Week 1-2: Conception and Implantation
The first two weeks of pregnancy are technically not part of the pregnancy at all, as they occur before a woman even knows she is pregnant. During this time, the egg and sperm unite to form a single cell called a zygote. This zygote begins to divide rapidly as it travels down the fallopian tube towards the uterus. Once it reaches the uterus, the zygote implants itself in the uterine lining, where it will continue to grow and develop over the next nine months.
Week 3-4: Embryonic Development Begins
During the third and fourth weeks of pregnancy, the zygote begins to differentiate into two distinct cell layers: the inner cell mass, which will become the embryo, and the outer cell layer, which will become the placenta. The embryo is still very small at this point, only about the size of a poppy seed, but it is already starting to develop the structures that will eventually become the heart, brain, and other organs.
Week 5-6: Major Organs Begin to Form
By week five, the embryo has grown to about the size of a sesame seed, and major organs such as the heart, liver, and kidneys are beginning to form. The embryo also develops tiny arm and leg buds, which will eventually become the arms and legs. By week six, the embryo’s heart is beating and beginning to pump blood.
Week 7-8: Fetal Period Begins
At around week seven, the embryo officially becomes a fetus. The fetus is now about the size of a blueberry and is beginning to look more like a human being. Facial features such as the eyes, ears, and nose are starting to take shape, and the fingers and toes are beginning to separate. By week eight, the fetus is about the size of a raspberry and is moving around inside the womb.
Week 9-10: External Genitalia Begin to Form
By week nine, the fetus is about the size of a cherry and is developing external genitalia. At this point, the sex of the baby can be determined through ultrasound. The fetus is also beginning to develop bones, and the muscles are becoming more defined. By week ten, the fetus is about the size of a strawberry and is starting to develop reflexes, such as the ability to swallow and suck.
Week 11-12: All Major Organs are Present
By week eleven, the fetus is about the size of a lime and is beginning to grow hair and eyebrows. All major organs are now present, and the fetus is beginning to produce urine. By week twelve, the fetus is about the size of a plum and is beginning to develop individual fingerprints. The fetus is also beginning to move more actively, although the mother may not be able to feel these movements yet.
Week 13-14: Rapid Growth and Development
During weeks thirteen and fourteen, the fetus undergoes a period of rapid growth and development. The fetus is now about the size of a peach, and the bones are becoming stronger. The baby’s skin is also becoming more transparent, allowing the blood vessels to be seen through the skin. By week fourteen, the fetus is about the size of a lemon and is beginning to develop a layer of fine hair called lanugo all over its body.
Week 15-16: Baby Begins to Hear
At around week fifteen, the fetus is about the size of an orange and is beginning to develop the ability to hear. The baby is also beginning to develop more coordinated movements, and the mother may be able to feel these movements. By week sixteen, the fetus is about the size of an avocado and is developing taste buds. The baby is also beginning to develop facial expressions, such as frowning and squinting.
Week 17-18: Baby Becomes More Active
During weeks seventeen and eighteen, the fetus becomes even more active, and the mother may be able to feel regular movements. The baby is also beginning to develop a protective coating called vernix, which helps to protect the skin from the amniotic fluid. By week eighteen, the fetus is about the size of a sweet potato and is beginning to develop more complex facial expressions, such as smiling and grimacing.
Week 19-20: Baby Develops Senses
At around week nineteen, the fetus is about the size of a mango and is developing its senses. The baby is now able to taste the amniotic fluid, and the eyes are beginning to develop more complex structures. By week twenty, the fetus is about the size of a banana and is beginning to develop more advanced brain function. The baby is also beginning to develop more distinct sleeping and waking patterns.
Week 21-22: Baby Grows Rapidly
During weeks twenty-one and twenty-two, the fetus undergoes another period of rapid growth. The baby is now about the size of a papaya and is beginning to develop more fat, which will help to regulate body temperature after birth. The baby’s skin is also becoming more opaque, and the eyelids are beginning to open and close. By week twenty-two, the fetus is about the size of a spaghetti squash and is beginning to develop more advanced lung function.
Week 23-24: Baby’s Lungs Continue to Develop
At around week twenty-three, the fetus is about the size of a grapefruit and is beginning to develop the ability to breathe. The baby’s lungs are producing surfactant, a substance that helps to keep the air sacs from collapsing. By week twenty-four, the fetus is about the size of a cantaloupe and is beginning to develop more advanced hearing. The baby may be able to hear the mother’s voice and other sounds from outside the womb.
Week 25-26: Baby’s Eyes Open
During weeks twenty-five and twenty-six, the fetus’s eyes are beginning to open more frequently, and the baby is becoming more responsive to light. The baby is also beginning to develop more complex taste preferences, and may prefer sweet or savory flavors. By week twenty-six, the fetus is about the size of a head of lettuce and is beginning to develop more advanced brain function, including the ability to form memories.
Week 27-28: Baby’s Nervous System Develops
At around week twenty-seven, the fetus is about the size of a cauliflower and is developing a more advanced nervous system. The baby is becoming more sensitive to touch, and may respond to tickling or poking. By week twenty-eight, the fetus is about the size of an eggplant and is beginning to develop more advanced motor skills. The baby is also beginning to develop more distinct facial features, such as eyebrows and eyelashes.
Week 29-30: Baby’s Lungs Mature
During weeks twenty-nine and thirty, the baby’s lungs are continuing to mature, and the baby is practicing breathing movements. The baby’s bones are also becoming stronger, and the baby is beginning to store more fat. By week thirty, the fetus is about the size of a large cabbage and is beginning to settle into a head-down position in preparation for birth.
Week 31-32: Baby’s Immune System Develops
At around week thirty-one, the fetus is about the size of a coconut and is developing a more advanced immune system. The baby is also beginning to develop more complex sleep patterns, and may spend up to 90% of the day sleeping. By week thirty-two, the fetus is about the size of a jicama and is beginning to develop more advanced hearing, including the ability to distinguish between different voices and sounds.
Week 33-34: Baby Continues to Grow
During weeks thirty-three and thirty-four, the baby continues to grow and gain weight. The baby is now about the size of a pineapple and is becoming more aware of its surroundings. The baby’s skin is also becoming smoother and less wrinkled, and the baby is beginning to develop more advanced taste preferences. By week thirty-four, the fetus is about the size of a butternut squash and is beginning to prepare for birth by settling even further into the pelvis.
Week 35-36: Baby’s Brain Develops Rapidly
At around week thirty-five, the fetus is about the size of a honeydew melon and is developing its brain at a rapid pace. The baby is also becoming more responsive to touch, and may respond to stroking or rubbing. By week thirty-six, the fetus is about the size of a papaya and is beginning to develop more complex facial expressions, such as smiling or pouting.
Week 37-38: Baby’s Lungs are Fully Developed
During weeks thirty-seven and thirty-eight, the baby’s lungs are fully developed, and the baby is practicing breathing movements more frequently. The baby is also gaining weight rapidly, and may be as much as seven pounds by the end of week thirty-eight. By this point, the baby is fully formed and is simply putting on the finishing touches in preparation for birth.
Week 39-40: Baby is Ready for Birth
At around week thirty-nine, the baby is fully developed and is simply waiting for the signal to begin the journey through the birth canal. By week forty, the baby is considered full-term, meaning that it is ready to be born at any time. The baby is now about the size of a watermelon and is fully prepared for life outside the womb.
Frequently Asked Questions
Q: Can a baby survive if it is born before the due date?
A: It depends on how premature the baby is and how well-developed its organs are. Babies born very prematurely may require medical intervention to help them breathe and regulate their body temperature.
Q: Can a mother influence the sex of her baby?
A: No, the sex of the baby is determined by the father’s sperm.
Q: Is it safe to have sex during pregnancy?
A: In most cases, yes. However, it is important to talk to your doctor about any concerns you may have and to avoid certain positions or activities that may be uncomfortable or risky.
Q: Can the baby feel emotions in the womb?
A: While it is unclear whether or not a fetus can experience emotions in the same way that a born baby can, research suggests that fetuses may be able to differentiate between different types of stimulation and may respond to things like music or the mother’s voice.
Q: What can I do to ensure a healthy pregnancy?
A: Eat a balanced diet, get plenty of rest, exercise regularly (with your doctor’s approval), avoid harmful substances like tobacco and alcohol, and attend all scheduled prenatal appointments.