Pregnancy Week By Week: How A Baby Develops

Pregnancy is an extraordinary journey filled with excitement, anticipation, and many physical and emotional changes. Over the course of approximately 40 weeks, both your body and your baby will undergo significant transformations. Each week of pregnancy brings new developments, from the moment of conception to the birth of your baby. Understanding what to expect week by week can help you prepare for the changes and challenges that come with pregnancy.

This guide will take you through pregnancy week by week, outlining the major milestones in your baby’s growth and the changes you may experience during each stage.

First Trimester (Weeks 1-12)

Week 1-2: Conception and Implantation

During the first two weeks of pregnancy, you’re technically not pregnant yet. Pregnancy is measured from the first day of your last menstrual period (LMP), even though conception typically occurs about two weeks later. Around the end of the second week, ovulation takes place, and the egg is fertilized by sperm, marking the official start of pregnancy.

Week 3: Fertilization

Once the egg is fertilized, it becomes a zygote, which begins to divide into more cells as it travels down the fallopian tube toward the uterus. This is the beginning of your baby’s development.

Week 4: Implantation

By week 4, the fertilized egg, now a blastocyst, attaches itself to the uterine wall in a process called implantation. This is when you might start noticing the first signs of pregnancy, such as a missed period.

Week 5: Early Development Begins

Your baby is now the size of a sesame seed, and the development of major systems begins. The neural tube, which will form the brain and spinal cord, is already in progress. At this stage, the embryo consists of three layers: the ectoderm, mesoderm, and endoderm, which will develop into all of the baby’s organs and tissues.

Week 6: Heartbeat Begins

One of the most exciting milestones occurs around week 6: your baby’s heart starts beating! This early heartbeat can sometimes be detected through an ultrasound. Other developments include the formation of tiny buds that will eventually become arms and legs.

Week 7: Limb Development

Your baby’s arms and legs continue to develop, and the facial features, including nostrils and the beginnings of the eyes, start forming. The brain and spinal cord are rapidly growing, and the baby is now about the size of a blueberry.

Week 8: Organs Begin to Form

At 8 weeks, the baby is officially called a fetus. Major organs, including the lungs, heart, and kidneys, are forming, though they are not fully functional yet. Fingers and toes start to develop from the hand and foot buds.

Week 9: Facial Features and Movements

Your baby is now around the size of a grape. The face is becoming more defined, with the formation of eyelids, a tiny nose, and a mouth. Though you won’t feel it yet, your baby is starting to move.

Week 10: Critical Development Phase

Most of the critical development is taking place by week 10. All of the baby’s major organs are now present, though they will continue to develop throughout pregnancy. The risk of congenital disabilities is now lower.

Week 11: Baby’s Growth Increases

By week 11, your baby is about the size of a lime. The baby’s fingers and toes are separating, and hair follicles and nails are beginning to grow. The development of the reproductive organs also starts during this time.

Week 12: End of the First Trimester

The first trimester ends at week 12, and by this point, your baby is fully formed. The organs will continue to mature, and your baby is now about 2.5 inches long. The risk of miscarriage decreases significantly at this stage.

Second Trimester (Weeks 13-26)

Week 13: Second Trimester Begins

Welcome to the second trimester! Many women find this trimester easier as nausea and fatigue often decrease. The baby’s bones are beginning to harden, and the intestines are developing within the abdomen.

Week 14: Baby Can Make Facial Expressions

By week 14, your baby can make facial expressions, and the kidneys are producing urine. You may also start showing a baby bump around this time.

Week 15: Hearing and Sensing Light

Your baby is growing quickly and is about the size of an apple. The baby can sense light and may start to hear muffled sounds from outside the womb.

Week 16: Quickening Begins

Around week 16, you may feel the first flutters of your baby’s movement, called quickening. The baby’s muscles and bones are developing further, and the head is now more erect.

Week 17: Fat Stores Begin

Your baby is gaining fat, which will help regulate body temperature after birth. The umbilical cord is now fully developed, supplying nutrients to the baby.

Week 18: Gender Can Be Identified

Around week 18, the baby’s genitalia can often be seen on an ultrasound, so if you wish to know the gender, this is the time to find out. Your baby’s bones are continuing to harden, and the ears are moving into their final position.

Week 19: Vernix Coats the Skin

The baby’s skin is covered in a waxy coating called vernix, which protects it from the amniotic fluid. The baby’s sensory development is increasing, and nerve cells for each of the five senses are developing.

Week 20: Halfway Mark

Congratulations! You’re halfway through your pregnancy. By week 20, the baby is about 6.5 inches long and weighs around 10 ounces. You may feel more frequent movements as your baby’s muscles grow stronger.

Week 21: Digestive System Development

Your baby’s digestive system is developing rapidly. The intestines are absorbing small amounts of sugar from the amniotic fluid, and the baby is practicing swallowing.

Week 22: Eyelids and Eyebrows Form

At week 22, your baby is about the size of a coconut. The eyelids and eyebrows are fully formed, and the baby is starting to practice breathing movements.

Week 23: Sense of Movement

Your baby can now feel your movements and may even respond to them by moving more. The baby’s skin is still wrinkled, but it will start to smooth out as fat continues to develop.

Week 24: Lungs Begin Maturing

The baby’s lungs are developing, and they’re producing surfactant, which will help them breathe after birth. The baby is also gaining weight rapidly.

Week 25: Hair Growth

By week 25, your baby’s hair is growing, and the baby now weighs around 1.5 pounds. The nervous system is continuing to develop, and the baby’s sense of touch is becoming more refined.

Week 26: Responding to Sounds

Your baby can hear your voice and may respond to sounds outside the womb. The eyes are also developing, and the baby may start to blink.

Third Trimester (Weeks 27-40)

Week 27: Third Trimester Begins

As you enter the third trimester, the baby’s growth is accelerating. The brain is developing rapidly, and the lungs are maturing in preparation for breathing.

Week 28: Kicking and Stretching

Your baby is now around 2.5 pounds and measures 14-15 inches. You may feel stronger kicks, stretches, and rolls as the baby continues to grow.

Week 29: Continued Brain Development

The baby’s brain is continuing to grow, and the bones are hardening even more. The baby can control body temperature and may even start dreaming during sleep.

Week 30: Gaining Weight

Your baby is continuing to gain weight and may now weigh around 3 pounds. The baby’s movements may feel more intense as the space in the uterus becomes tighter.

Week 31: Increased Awareness

By week 31, your baby is becoming more aware of their surroundings. The baby can open and close its eyes and is responsive to light and sound.

Week 32: Head Down Position

Most babies will start moving into the head-down position in preparation for birth. The baby’s skin is becoming less wrinkled as fat accumulates beneath the surface.

Week 33: Practicing Breathing

Your baby is practicing breathing by inhaling and exhaling amniotic fluid. The baby may also start to hiccup, which you might feel as rhythmic movements in your belly.

Week 34: Final Stages of Development

As your baby approaches full term, the lungs, brain, and nervous system are nearing full development. The baby’s bones are fully formed, but they are still soft and pliable for delivery.

Week 35: Increased Fat Storage

Your baby is now putting on the finishing touches in terms of weight gain. Fat is accumulating under the skin, making the baby look plumper and healthier.

Week 36: Preparing for Birth

Your baby is almost ready to be born! Most babies born at this stage can survive without major medical intervention. The baby’s head may engage, or drop into your pelvis, in preparation for birth.

Week 37-40: Full Term

From week 37 onward, your baby is considered full term. The baby continues to gain weight and may weigh between 6 to 9 pounds by the time of birth. Your baby’s organs are fully developed, and the baby is ready to be bornPregnancy is one of the most transformative experiences in a woman's life. From conception to the final weeks before birth, understanding the week-by-week development can be immensely helpful. Each week, your body adapts to accommodate your growing baby, and both mother and child go through numerous physical and emotional changes.

For a week-by-week pregnancy guide that captures all the crucial milestones, understanding the changes from conception to birth can offer insights into the incredible journey you're embarking on.