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Your Baby’s Development and Growth, Week-by-Week

Welcoming a new life into the world is an awe-inspiring experience for any parent, and from the moment you find out you’re pregnant, understanding your baby’s development becomes a top priority. Your baby will grow and change in incredible ways over the course of your pregnancy, transitioning from a tiny bundle of cells to a fully-formed human being. And after you give birth, you’ll continue to track your baby’s development milestones alongside your healthcare provider, oftentimes with the help of a growth chart.

In this post, we’ll walk you through your baby’s development in each trimester, week by week. You’ll also learn what growth charts are, how to get one, and how you can tell if there is a problem with your baby’s growth. 

Fetal Development

Before you even know you’re pregnant, the process of fetal development has begun. Your baby will go through three main stages of development between conception and birth: germinal, embryonic, and fetal. 

Stages of fetal development

Germinal stage: This first stage of fetal development is also the shortest. It starts at conception, when the sperm fertilizes the egg in your fallopian tube, forming a zygote. Next, the zygote moves down your fallopian tube toward the uterus, dividing many times until it creates two separate structures (one will become the embryo, while the other will become the placenta).

The tiny ball of cells that will later develop into the embryo is now inside a structure called the blastocyst. The blastocyst implants into the lining of the uterus, which triggers your body to start producing hormones to support a pregnancy.

Embryonic stage: From weeks 3-8 of pregnancy, the blastocyst develops human characteristics and is now called an embryo. Your baby’s head, eyes, mouth, and limbs form during this time, along with the neural tube, which later will become the brain and spinal cord. By the end of this stage, most of your baby’s organs and life-sustaining systems will have started to develop, including their heartbeat. 

Fetal stage: Around week 9 of pregnancy, the embryo officially turns into a fetus. Thus begins the fetal stage, which lasts until birth and is when major fetal weight growth occurs. Your baby’s major organs (including their sex organs) grow and mature during this time, as well as fingernails, eyelashes, and hair. 

Stages of Baby Growth by Trimester

You and your baby will experience big changes throughout each trimester. Read on to discover the nitty-gritty of your baby’s growth and development, week by week. 

First Trimester

Weeks 1-2: Believe it or not, you’re actually not pregnant the first week or two of the time period that’s considered your pregnancy term. Conception usually occurs around two weeks after your last period begins, and your healthcare provider calculates your estimated due date by counting 40 weeks from the beginning of your last period. 

Week 3: This is when conception actually happens, through fertilization—sperm and egg come together in your fallopian tube to form a zygote (or multiple zygotes, if more than one egg is fertilized or the egg splits). 

Week 4: The tiny ball of cells, dividing rapidly and now referred to as a blastocyst, initiates the process of implantation by burrowing into the uterine lining. 

Week 5: The neural tube forms, which eventually will be the foundation for your baby’s brain and spinal cord. 

Week 6: Blood cells are starting to form, circulation begins, and your healthcare provider may be able to detect a heartbeat on an ultrasound. Small buds that will become your baby’s arms and legs form, as well as the structures that will eventually turn into their eyes, ears, and mouth. 

Week 7: Your baby’s brain and face are growing—their retinas start to form and depressions that will later become nostrils are now visible. 

Week 8: By the end of week 8, the embryo is now considered a fetus, and it’s about 0.5 to 1 inch long. Eyes become visible, ears start to form, and the fetus has webbed hands and feet.

Week 9: Arms grow and elbows and toes appear. Your baby’s body shape starts to take on more of a human appearance, although its head is still 50% of its length. 

Week 10: No more webbing, as the arms, hands, fingers, feet, and toes have fully formed. Your baby can now bend their elbows, and their eyelids and external ears continue to form.

Week 11: Your baby’s external genitals start forming, and buds where teeth will develop appear. While it’s too soon to feel any kicks, your baby is starting to explore movement through things like opening and closing their fists and mouth.

Week 12: Your baby is growing fingernails, and at this point, all their organs, limbs, bones, and muscles have started developing. By the end of the first trimester, the fetus is roughly 2.5 to 3 inches long. 

Second Trimester

Week 13: Your baby’s vocal cords form and their head starts to become more proportionate to the rest of their body. Some of your baby’s bones start to harden, particularly in the skull and long bones.

Week 14: External genitals fully develop this week, making your baby’s sex more apparent. Their skin begins to thicken and fine hair starts growing.

Week 15: Lungs start to develop, and some organs, such as the intestines and ears, move to their permanent position. Your baby is starting to smile! 

Week 16: While their movements are too slight to be detected by you at this point, your baby’s limb movements are becoming coordinated and can be detected via ultrasound. At this point the fetus is roughly 5 inches long and weighs around 4 ounces.

Week 17: Your baby starts to put on fat and their toenails develop

Week 18: Your baby’s ears begin to stand out on the side of their head and they may start to hear sounds. At this stage, your baby’s digestive system has started working. 

Week 19: Your baby can hiccup! They are getting stronger, and most moms start to feel them kicking. Plus, a coating called vernix caseosa begins to cover your baby, which helps protect their skin from exposure to amniotic fluid.

Week 20: This week marks halfway into your pregnancy. The area of the brain housing your baby’s five senses starts developing, and the fetus is roughly 9 to 10 inches long and weighs around 1 pound.

Weeks 21-24: Your baby’s kicks and turns grow stronger and their sucking reflex is developing, enabling them to suck their thumb. The beginning signs of your baby’s fingerprints and footprints appear in the form of ridges on their hands and feet. By the end of week 24, the fetus is about 12 inches long and weighs about 2 pounds.

Weeks 25-28: Lungs have fully formed and begin to make surfactant, a substance necessary for breathing after birth. Your baby’s eyelids can open and close and their skin starts to look smoother as they develop more fat. At the end of the second trimester, the fetus is around 14 to 15 inches long and weighs 2 to 3 pounds.

Third Trimester

Weeks 29-32: Your baby can now stretch, kick, and make grasping movements. Their eyes can sense changes in light, their bone marrow is creating red blood cells, and for boys, the testicles start to descend. The fetus is 17 to 18 inches long and weighs up to 5 pounds.

Weeks 33-36: Your baby’s bones harden, although the skull stays flexible. Their limbs start looking chubby. Their brain continues to grow and is about two-thirds of the weight it should be at birth. Most of the baby’s growth spurts occur in the third trimester! The fetus is around 17 to 19 inches long and weighs between 6 and 7 pounds.

Weeks 37-40: In the final weeks before your due date, your baby’s circulatory and musculoskeletal systems finish developing. Their lungs, brain, and nervous system are wrapping up their development, too. The fetus is around 18 to 20 inches long and weighs between 7 to 9 pounds.

What Are Growth Charts and How Do I Get One?

Once you give birth to your baby, you will continue to monitor their growth, especially in the first year. While exploring the question “How can I follow my baby’s development?”, you may have heard about growth charts. 

Growth charts are tools used by healthcare providers to track a child’s growth and development over time. They are graphical representations of how measurements, such as weight, length, and circumference of your baby’s head, compare to those of other babies of the same age and sex. Growth charts are important to assess whether a baby is growing at an appropriate rate and to identify any potential growth concerns.

You can obtain a growth chart from your healthcare provider, who will regularly measure your baby’s growth during check-ups. These measurements are usually plotted on the growth chart to track your baby’s growth trajectory. This is important because, according to pediatric hospitalist Dr. Liz Donner, MD, “A single plot on the growth chart doesn’t tell us much; we really need to see the trend over time.”

Growth charts are typically based on population data and percentiles, which indicate what percentage of babies of the same age and sex fall into a similar range of measurements. For example, if your baby’s weight falls on the 50th percentile, it means that 50% of babies of the same age and sex weigh the same or less, and 50% weigh more.

It’s important to remember that your baby’s growth chart is just one tool for monitoring growth and development and should not be the sole diagnostic instrument to determine your baby’s health. Babies come in a wide range of healthy shapes and sizes, and your healthcare provider will interpret the growth chart in the context of your baby’s overall health, medical history, and other factors to assess your baby’s growth accurately.

What is Ideal Baby Growth?

Every baby’s growth and development is unique, but there are some common milestones you can look towards to assess how your baby’s doing. On average, by their first birthday, most babies triple their birth weight, grow around 9.5 inches longer, and their heads grow 4 to 5 inches. But it’s important to remember that a baby’s growth is not constant—it comes in bursts. 

Dr. Liz Donner, MD says other developmental milestones parents can expect to see in the first year of life include, “smiling at caregivers by 2 months, holding a toy by 4 months, rolling from tummy to back by 6 months, sitting without support by 9 months, and cruising along furniture by 12 months of age.”

How can I tell if there is a problem?

Some things that could indicate a problem in your baby’s size or development include:

  • If your baby is small for gestational age (SGA), which describes a fetus that is smaller than is typical for the number of weeks of pregnancy. Most SGA babies are small due to fetal growth problems that occur during pregnancy, and they can experience problems at birth including decreased oxygen levels, low blood sugar, and difficulty maintaining a normal body temperature. If you have an SGA baby, your healthcare provider will likely  be able to diagnose this before you give birth, using technology such as an ultrasound or Doppler flow.
  • If your baby has slow weight gain, to the point that it interferes with healthy development. According to the Boston Children’s Hospital, slow weight gain may be a problem if, “your newborn doesn’t regain their birth weight within 10 to 14 days after their birth, your baby up to 3 months old gains less than an ounce per day, your infant between 3 and 6 months gains less than 0.67 ounces a day, or your child of any age has been growing steadily and suddenly stops growing”. 
  • If your child’s weight or height percentile deviates from its normal pattern. This doesn’t always signify a problem, especially during infancy when it’s more common for children to show changes in growth percentiles. But double-check with your healthcare provider if your child’s no longer following their usual growth pattern (i.e. their height or weight has a dramatic drop or increase in percentile).

When in doubt, talk to your doctor 

It’s easy to grow fearful when assessing your baby’s growth, both during your pregnancy and after giving birth. So make sure to share your concerns with your healthcare provider—it will enable you to get the information you need, have your fears assuaged, and enjoy all the beautiful parts of watching your little one grow and develop.