Whether you’re a new mom, a mother-to-be, or someone who’s interested in having kids, you’re probably curious about how babies transition from small embryos to full-grown infants.
When do they develop their brains, lungs, and other organs and produce their first heartbeat?
How big are they during the first, second, and third trimesters? Can they really hear their mother’s voice? These are just some of the questions that might be floating in your mind.
To answer these questions, we’ve created a guide on monthly fetal growth and development. Check each month as you go along your pregnancy to know what’s going on in your womb and track your baby’s progress.
This is when most of the action happens. Once your egg gets fertilized by a sperm, it will multiply into many cells and becomes a blastocyst while it travels down your Fallopian tube and to your uterus.
During the third week, the blastocyst will attach itself to your uterine lining, which would become its source of nutrients over the next nine months.
By the fourth week, the blastocyst becomes an embryo and becomes the size of a poppy seed. During this time, it starts to form the face and the neck and begins the development of the lungs, heart, liver, stomach, and circulatory system.
Your baby is as small as a sesame seed at the start of the second month and grows to the size of a kidney bean at the end.
His heart will start beating in the fifth week of pregnancy, while the rest of his organs and limbs continue to develop. In the eighth week, he will start to move around, although you won’t feel any movement yet.
At the beginning of this month, he’ll be around the size of a grape, and then become as large as a lime four weeks later.
His basic anatomy and physiology are in place by week 9, and he completes the critical part of the development by week 10.
From here on out, he only has to continue growing and gaining weight — which he rapidly does over the following weeks.
He also gets more active as he starts to kick and stretch, and by the time the 12th week rolls around, he’ll develop his reflexes and start to move his fingers, making sucking motions with his mouth.
This is when you enter the second trimester. It’s a good time for you since your fatigue and morning sickness will have disappeared by now.
It’s also a great time for your baby since he’s now more developed and miscarriage is less likely to happen.
During this phase, he will grow from as small as a pea pod to as large as an avocado by the end of the 16th week. He’ll start to suck his thumb, sense light, and even kick!
If you get an ultrasound at the end of the fourth month, the results might indicate the sex of your baby.
This is the phase when you can begin to interact with your baby. You’ll get to feel his movements as he starts to flex his arms and legs.
He, in turn, can hear your voice and other sounds as his senses become even more developed. You’ll want to play nursery rhymes, read children’s books, or simply talk to him about how excited you are to meet him.
At this stage, he will grow from being turnip-sized to being as big as a banana.
Growth and development continue as your baby gets more distinct facial features and progresses to giving harder kicks that you’ll definitely notice.
He’ll also become more adept at the hearing, and the sounds he hears now might be imprinted on him so much that he recognizes them after birth.
He starts out as small as a carrot on the 21st week and grows to become as big as an ear of corn in the 24th week.
This is the time when your baby starts to fill out by developing baby fat, which will smooth out his wrinkly skin and makes him resemble a chubby newborn.
His hair starts to come in, covering not just his head but also his entire body in the form of soft, fine hair that medical experts call lanugo.
His lungs become stronger as he learns to inhale and exhale (breathing in the amniotic fluid that surrounds him), while his eyesight develops further as he learns to blink and becomes more sensitive to light.
He also starts to sleep and develops a sleeping and waking schedule. He’s around the size of standard rutabaga at the start of the month and becomes as big as a large eggplant in week 28.
Your baby gains one-third to even half his birth weight at this stage onwards, growing to around the size of a pineapple at the end of the 32nd week.
This is why you’ll notice rapid weight gain when the eighth and ninth months come along.
Your baby’s head grows bigger to accommodate his developing brain, and he gains more subcutaneous fat, especially on his arms and legs. He also starts to move his head from side to side.
During the last month of pregnancy, your baby finishes up his development as his central nervous system, as well as other organs and systems, become more mature.
He starts to lose the fine down hair that covered his entire body, retaining the hair on his scalp.
He gains around an ounce every day, growing from a cantaloupe-sized fetus to a newborn infant who’s as big as a leek.
His brain and lungs are not yet fully developed, even during the 37th week, although he can survive in the outside world if you give birth a few days earlier than your due date.
Pregnancy might be full of challenges, but it’s important to remember that it’s also an exciting time.
By tracking your baby’s progress in your womb, you’ll be more aware of the milestones he achieves and knows what you need to do to support his growth and development.
- C. Mayer K. S. Joseph. Fetal growth: a review of terms, concepts and issues relevant to obstetrics. 30 May 2012. [Link]
- Janet A. DiPietro. Baby and The Brain: Advances in Child Development. Annual Review of Public Health 2000 21:1, 455-471. [Link]
- Galjaard, S., Devlieger, R. & Van Assche, F. (2012). Fetal growth and developmental programming. Journal of Perinatal Medicine, 41(1), pp. 101-105. Retrieved 28 Sep. 2019, from [Link]