Understanding Your Baby's Development During Pregnancy: Week by Week

Pregnancy is a miraculous journey filled with anticipation, excitement, and countless questions, especially for first-time parents. One of the most awe-inspiring aspects of pregnancy is the rapid development of your baby, who starts as a tiny cluster of cells and, in just 40 weeks, grows into a fully formed human being. Understanding your baby’s development during pregnancy, week by week, can help you appreciate the incredible changes happening inside you and provide reassurance as you prepare for the arrival of your little one.

This guide takes you through the journey of pregnancy, exploring the fascinating changes that occur each week. From the moment of conception to the final weeks leading up to birth, we'll delve into the key milestones of fetal development, providing insights into what’s happening inside your womb.

Weeks 1-4: Conception and Early Development

Week 1-2: Preconception and Fertilization

Pregnancy is counted from the first day of your last menstrual period (LMP), even though actual conception usually occurs about two weeks later. During the first two weeks, your body is preparing for ovulation and fertilization. Around the end of the second week, an egg is released from your ovary, and if sperm is present, fertilization can occur, resulting in the formation of a zygote.

Week 3: Fertilization and Implantation

Once fertilized, the zygote begins dividing rapidly as it travels down the fallopian tube toward the uterus. By the time it reaches the uterus, it has developed into a blastocyst, a tiny ball of cells that will eventually form the embryo and placenta. Around the end of this week, the blastocyst implants itself into the uterine lining, marking the beginning of pregnancy.

Week 4: Formation of the Embryo

As implantation occurs, the blastocyst begins to develop into an embryo. This week marks the start of the embryonic period, during which all of your baby’s major organs and systems begin to form. The embryo is now about the size of a poppy seed, and the cells are differentiating into different layers that will give rise to the baby’s body parts.

Weeks 5-8: Early Embryonic Development

Week 5: The Heart Begins to Beat

During the fifth week, your baby’s heart begins to form and beat for the first time, though it’s still too early to detect on an ultrasound. The neural tube, which will become the brain and spinal cord, also starts to develop. At this stage, your baby is about the size of a sesame seed.

Week 6: Formation of Facial Features and Limbs

By the sixth week, your baby’s facial features begin to take shape, including tiny indentations that will become the eyes and ears. Limb buds, which will develop into arms and legs, also start to form. The heart is now beating at a regular rhythm, and the neural tube is closing. Your baby is about the size of a lentil.

Week 7: Growth of Major Organs

This week, your baby’s brain is growing rapidly, and the development of major organs, including the lungs, liver, and kidneys, is underway. The limb buds are growing longer, and the hands and feet are beginning to form, though they look more like paddles at this stage. Your baby is now about the size of a blueberry.

Week 8: Beginning of Bone Formation

By the eighth week, your baby’s bones are starting to form, replacing the earlier cartilage. The ears are beginning to take shape both inside and outside, and the eyes are becoming more defined. The digestive system is also developing, with the intestines starting to form. Your baby is now about the size of a kidney bean.

Weeks 9-12: Transition to Fetal Stage

Week 9: Development of Reproductive Organs

During the ninth week, your baby’s reproductive organs begin to develop, though it’s still too early to determine the sex on an ultrasound. The arms and legs are growing longer, and the baby is beginning to make small movements, though you won’t feel them yet. The heart has divided into four chambers and is beating at about 170 beats per minute. Your baby is now about the size of a grape.

Week 10: Formation of Vital Organs

At week 10, all of your baby’s vital organs are fully formed and are beginning to function, though they will continue to mature throughout the pregnancy. The baby’s head is still large compared to the rest of the body, but the neck is developing, allowing for more movement. The eyes are closed and will remain shut until around week 27. Your baby is now about the size of a kumquat.

Week 11: Development of Fingernails and Tooth Buds

This week, your baby’s fingernails and tooth buds start to form. The bones are continuing to harden, and the baby is practicing swallowing and breathing movements. The kidneys are now producing urine, which is released into the amniotic fluid. Your baby is about the size of a fig.

Week 12: End of the First Trimester

By the end of the twelfth week, your baby has completed the most critical part of development, and the risk of miscarriage drops significantly. The baby’s face is becoming more human-like, with a well-formed nose and chin. The baby is also able to open and close its fists and is starting to produce digestive enzymes. Your baby is now about the size of a lime.

Weeks 13-16: Rapid Growth and Development

Week 13: Second Trimester Begins

As you enter the second trimester, your baby’s organs continue to mature, and the baby starts to grow more rapidly. The intestines are moving into the abdomen from the umbilical cord, where they developed earlier. The baby is now about the size of a pea pod.

Week 14: Development of Facial Muscles and Hair

By the fourteenth week, your baby’s facial muscles are becoming more refined, allowing for expressions such as frowning or squinting. Lanugo, a fine hair, begins to cover the baby’s body, providing warmth and protection. Your baby is about the size of a lemon.

Week 15: Hearing Begins to Develop

This week, your baby’s hearing starts to develop, and they may be able to hear your heartbeat and voice, although the sounds are muffled. The baby’s bones are continuing to harden, and the legs are now longer than the arms. Your baby is about the size of an apple.

Week 16: Movement Becomes More Coordinated

At 16 weeks, your baby’s movements become more coordinated, and you may start to feel the first flutters, known as quickening. The baby is also developing more muscle tissue and fat, and the heart is pumping about 25 quarts of blood each day. Your baby is now about the size of an avocado.

Weeks 17-20: Mid-Pregnancy Milestones

Week 17: Fat Accumulation Begins

Your baby is starting to accumulate fat, which will provide energy and help regulate body temperature after birth. The baby’s skeleton is transforming from soft cartilage into bone, and the sweat glands are beginning to develop. Your baby is now about the size of a turnip.

Week 18: Nervous System Development

At 18 weeks, your baby’s nervous system is rapidly developing, with myelin (a protective covering) forming around the nerves. This process, called myelination, will continue well into childhood. The baby’s movements are becoming more purposeful, and the uterus is now large enough that you might start to show more visibly. Your baby is about the size of a bell pepper.

Week 19: Vernix Caseosa Forms

This week, your baby’s skin is beginning to be coated with vernix caseosa, a waxy substance that protects the skin from the amniotic fluid. The baby’s senses are also developing, with taste buds, smell receptors, and touch receptors becoming more sensitive. Your baby is now about the size of a mango.

Week 20: Halfway Mark

Congratulations—you’re halfway through your pregnancy! At 20 weeks, your baby’s limbs are proportionate to the rest of the body, and they are practicing more complex movements like sucking and grasping. This is also the time when many parents find out the baby’s sex during the mid-pregnancy ultrasound. Your baby is now about the size of a banana.

Weeks 21-24: Sensory and Cognitive Development

Week 21: Taste Buds and Swallowing

Your baby’s taste buds are fully developed by week 21, and they may start to taste the flavors of the food you eat as they swallow amniotic fluid. The baby is also practicing more swallowing, which helps develop the digestive system. Your baby is now about the size of a carrot.

Week 22: Development of Sleep Patterns

At 22 weeks, your baby starts to develop sleep-wake cycles, and you may notice patterns of activity and rest. The baby’s brain is continuing to develop rapidly, and the senses of sight, hearing, smell, and touch are becoming more refined. Your baby is now about the size of a spaghetti squash.

Week 23: Lung Development

The baby’s lungs are developing more alveoli, the tiny air sacs that will allow for breathing after birth. However, the lungs are not yet fully functional, and the baby is still practicing breathing movements by inhaling amniotic fluid. Your baby is now about the size of a large mango.

Week 24: Viability and Continued Growth

At 24 weeks, your baby is considered viable, meaning they have a chance of survival if born prematurely, though intensive medical care would be needed. The baby’s organs, including the lungs, brain, and eyes, continue to mature. Your baby is now about the size of an ear of corn.

Weeks 25-28: Preparing for Birth

Week 25: Development of the Nervous System

Your baby’s nervous system is maturing, with more connections being made between the brain and muscles. The baby is also responding to external stimuli, such as light and sound, and you may feel more pronounced movements. Your baby is now about the size of a rutabaga.

Week 26: Continued Lung Development

The lungs are producing surfactant, a substance that will help the baby’s lungs expand after birth. The baby’s eyes are opening, and they can respond to bright light by moving away from it. Your baby is now about the size of a head of lettuce.

Week 27: Third Trimester Begins

As you enter the third trimester, your baby is growing rapidly and gaining weight. The baby’s brain is developing more grooves and folds, and the eyes can now blink. Your baby is now about the size of a cauliflower.

Week 28: Brain and Organ Maturity

By 28 weeks, your baby’s brain is nearly fully formed, and the lungs are continuing to mature. The baby is also starting to produce more body fat, which will help regulate body temperature after birth. Your baby is now about the size of an eggplant.

Weeks 29-32: Rapid Growth and Weight Gain

Week 29: Muscle and Fat Accumulation

Your baby is gaining muscle and fat at a faster rate, which is helping them grow stronger. The bones are hardening, but the skull remains soft to make childbirth easier. Your baby is now about the size of a butternut squash.

Week 30: Development of the Immune System

The baby’s immune system is developing, and they are receiving antibodies from you that will help protect them after birth. The baby is also practicing breathing movements more regularly. Your baby is now about the size of a cabbage.

Week 31: Increased Movement and Positioning

At 31 weeks, your baby’s movements may feel more intense as they continue to grow and run out of space in the womb. The baby is likely getting into a head-down position in preparation for birth. Your baby is now about the size of a coconut.

Week 32: Final Stages of Organ Development

Your baby’s organs are nearing full maturity, and the lungs are producing more surfactant. The baby’s skin is becoming less transparent as more fat accumulates. Your baby is now about the size of a jicama.

Weeks 33-36: Final Preparations for Birth

Week 33: Continued Brain Growth

The baby’s brain is continuing to grow and develop, with more connections being made. The baby is also gaining more fat, which will help keep them warm after birth. Your baby is now about the size of a pineapple.

Week 34: Lung Maturity

By 34 weeks, the baby’s lungs are almost fully developed, and they would likely be able to breathe on their own if born prematurely. The baby’s movements may become more restricted due to the limited space in the womb. Your baby is now about the size of a cantaloupe.

Week 35: Gaining Weight and Getting Ready

Your baby is continuing to gain weight, adding about half a pound each week. The baby’s head may engage or drop into the pelvis, which is known as lightening. Your baby is now about the size of a honeydew melon.

Week 36: Final Growth Spurt

At 36 weeks, your baby is nearly fully developed, and their body is preparing for life outside the womb. The baby’s movements may decrease slightly as they run out of space, but you should still feel regular kicks and rolls. Your baby is now about the size of a papaya.

Weeks 37-40: The Final Countdown

Week 37: Full-Term

Your baby is now considered full-term, meaning they are fully developed and ready for birth. The baby’s skin is becoming smoother, and they are continuing to gain weight. Your baby is now about the size of a watermelon.

Week 38: Final Preparations

During the final weeks of pregnancy, your baby is shedding the vernix caseosa and lanugo, which will be ingested and become part of the first bowel movement after birth. The baby is also storing iron in the liver to be used after birth. Your baby is now about the size of a pumpkin.

Week 39: Getting Ready for Birth

At 39 weeks, your baby is ready for birth. The lungs are fully mature, and the baby’s body is producing more hormones that will help with the transition to life outside the womb. Your baby is now about the size of a small watermelon.

Week 40: Birth

Congratulations! You’ve reached the 40-week mark, and your baby is ready to meet the world. The baby is likely in a head-down position and may have dropped lower into your pelvis. Your baby is now about the size of a large watermelon, and labor could start at any moment.

Conclusion

Understanding your baby’s development during pregnancy, week by week, is a journey of discovery and wonder. Each week brings new milestones and changes as your baby grows and prepares for life outside the womb. By staying informed about these developments, you can better connect with your baby and make informed decisions about your pregnancy and birth.

As you prepare for the arrival of your little one, remember that every pregnancy is unique, and your baby’s development may vary slightly from the typical timeline. Trust your body, stay connected with your healthcare provider, and enjoy the incredible experience of bringing a new life into the world.