Month-by-Month Guide to Your Baby’s Development During Pregnancy – The Baby Cubby

Pregnancy can be one of the most exciting times in a parent’s life. Unfortunately, for many, it can also bring a lot of challenges and anxieties. For me, I always struggled with pregnancy. From feeling self-conscious to all those aches and pains, I had an extremely hard time feeling positive about pregnancy. That’s why it was so essential for me to find things that helped me feel more positive and to look forward to the little baby at the end of the long road.

One of the things that helped me the most was keeping track of my baby’s development each month! Not only did it help me feel like things were progressing despite the feeling that pregnancy just dragged on, but it also helped me connect to that little baby I couldn’t yet see.

So whether you just want to start building that bond with your baby even before birth or you are just curious about what in the world is happening in there, here’s your guide to your little one’s development during pregnancy!

Pregnancy Timeline Explained

Starting out, it’s important to understand just how the timeline for pregnancy plays out. When going to your first prenatal appointment, you may notice that your healthcare provider will ask you when you last had your period. This is because your official pregnancy “start date” comes before your baby is even conceived. In fact, the first day of your last period is when your pregnancy officially begins. For the sake of this article, when I refer to a developmental stage in “weeks”, I’m going off of this official start date (even though your baby was technically conceived about 2 weeks later).

The reason why providers go off of this date is because it has been shown to give the most accurate indicator of when you will deliver, as knowing when a woman ovulated or the exact moment they conceived is pretty difficult to calculate. However, since every woman’s menstrual cycle is different, it’s important to remember that it’s not an exact science. That’s why very few women deliver on their actual due date (less than 5%!) So try to remember this throughout your pregnancy and don’t rely too much on the specific date.

In general, though, women are pregnant for 40 weeks or 280 days. This equates to about 9 months. When pregnant with my first, this confused me a bit. If most months have about 4 weeks in them, that would make pregnancy 10 months long. And some online guides will define each “month” as 4 weeks, giving you a 10 month pregnancy timeline.

We know this isn’t entirely accurate though, because each month of the year varies–some have 4 weeks, while most months are a bit longer than 4 weeks. In order to simplify things, I’m going to keep things a bit more general and discuss the basics of what’s happening each month (especially because babies will develop at slightly different rates.) Some of my “months” will technically have 5 weeks in them. This allows me to divide the months in a way that makes the most sense developmentally. 

Another note about pregnancy timelines to keep in mind: there will be some slight variations in what’s considered a trimester among experts. Some refer to the first trimester as the first 12 weeks, while some say it lasts until the start of week 14. In this article, I will be going by the widely accepted rule of 0 through the end of week 13 for the first trimester, 14 through the end of 27 weeks for the second, and 28 to 40 weeks for the third. 

So with all that in mind, here’s your month-by-month guide to your baby’s development during pregnancy!

First Trimester

Month 1 (Weeks 1 through about 5)

As I said before, pregnancy technically begins before conception! So the first week of pregnancy, you will actually be on the last period you’ll have for a while. Sometime during the second week, you will ovulate. It is at this time that you should have sex if you’re actively trying to conceive. Having sex before you think ovulation will occur will give you the best chance to conceive, as sperm can survive up to 5 (sometimes more) days inside the body, but once the egg is released, it is only viable for about 24 hours. If the sperm finds the egg, fertilization has officially occurred, and what is called a “zygote” will form!

Next (during week 3), the fertilized egg will travel to its ultimate destination-the uterus- where it will then implant into the wall of the uterus. As it makes its journey, the cells are constantly dividing, and the “blastocyst” forms. As the blastocyst starts implanting in the uterus, it divides into 2 parts-half of which becomes the embryo, and the other half will form the placenta. The amniotic sac will also form around your baby, which will help protect your little one during the next 9 months!

The embryo will then divide into 3 layers: the ectoderm, which will form the skin, nervous system, eyes, and ears; the mesoderm, which will become the heart as well as the skeletal system, kidneys, and reproductive system; and the endoderm, which becomes the intestines and lungs. One of the first things that will develop from this division is the neural tube, which will eventually become the brain and spinal cord.

Near the end of the first month, implantation becomes complete, and you now have an embryo! You have probably missed your period by this point as well, as implantation signals to your body that you’re pregnant. This leads to a cascade in changes in your hormones, and you may start experiencing some of your first pregnancy symptoms.

Although it may seem like not a lot is happening in this first month since about half of it is just building up to conception, many vital things are happening! As soon as the embryo forms, a lot of vital parts begin to develop! That’s why it is so important that you prepare for a healthy pregnancy even before you get pregnant by learning all that you can.

One of the best things you can do for both you AND your future baby is to start taking a quality prenatal vitamin at least a month before trying to conceive. This will keep you strong while also helping your baby to develop properly as well.

Month 1 Overview:

• Conception occurs & a zygote forms
• Cells start to divide, blastocyst forms
• Blastocyst implants in the uterus
• Division into placenta and embryo occur
• Hormonal changes occur, leading to missed period
• Embryo divides into 3 layers: the ectoderm, mesoderm, and endoderm
• Neural tube forms

Baby’s size at the end of 5 weeks: Sesame seed

Photo Source: Baby Center

Month 2 (Weeks 6 through 9)

Once the embryo develops in month 1, big things continue to happen! It’s at this time that the neural tube closes, giving rise to brain and spine development! At the same time, other major organs, such as the cardiovascular, respiratory, and digestive organs, start to develop. As the heart begins to develop, it will start to make pumping motions, which can sometimes be seen on ultrasound!

It is also in month 2 that your baby starts to look much more like a baby! The embryo starts to take on a C-shape and little tiny bumps form that will soon become arms and legs. In weeks 7 through 9, especially, the head, facial features, and limbs will become more prominent. By the end of month 2, those little frog-like hands and feet now have obvious fingers and toes, and the ears, eyes, nose, and tiny lips have begun to form. 

With all the rapid organ development, this is again why taking a prenatal before getting pregnant is so important!

Month 2 Overview:

• Neural tube closes off and will become the brain and spine
• Heart and other major organs continue to develop
• Embryo takes on C-shape, but begins to straighten out by end of week 9
• Arms and legs form, fingers and toes develop
• Facial features become more obvious-ears, eyes, nose, and lips form
• “Tail” starts to disappear
• Umbilical cord is fully formed and begins providing baby with all it needs

Baby’s size at the end of 9 weeks: About the diameter of a penny in length, a piece of macaroni, or the size of grape

Photo Source: Family Education

Month 3 (Weeks 10-13)

Up until this point, your baby’s head has been the biggest part of their little body. In month 3, however, their body will grow, and their head will start to become a little more round. The fingers and toes lose their webbing, and your baby’s elbows begin to function, allowing for bending of the arms. By the end of month 3, the finger and toenails will also start to grow.

Their facial features also become even more prominent than they were last month, as their ears continue to take shape on the sides of their head. Your baby will also start to open and close his or her mouth (where little teeth are forming under the gums!)

When it comes to organ development, bile is beginning to be synthesized in the liver. Blood cells are formed, and begin to circulate, and urine is beginning to be produced as well by the kidneys. The muscular and skeletal systems continue to develop as bones form from cartilage, and the intestines have formed within the abdomen, where in just a few months they will be used to process their breastmilk or formula.

Another notable development that occurs in month 3 is the external genitalia start to form! However, it is often too soon to determine the sex of the baby on an ultrasound. This is because, at this point in time, the developing genitals of both female and males are still very tiny, and the clitoris and penis growth looks fairly similar. So it’s best to wait until the 20 week anatomy scan for a more definitive answer (unless you do a blood test that reveals the gender for you!)

It’s also in month 3 when your little embryo becomes a fetus, and your risk of miscarriage drops drastically!

Month 3 Overview:

• Your little one is now a fetus
• Baby’s body size is increasing, so head and body are a bit more proportional
• Fingers and toes lose webbing, and fingernails start to form
• Facial features become more prominent, ears are now on the side of the head
• Blood cells are synthesized and begin to circulate
• Bile is being synthesized by the liver
• Urine is formed
• Intestines are in the abdomen now
• Bones start to harden
• Teeth are beginning to form
• External genitalia forms

Baby’s size at the end of 13 weeks: Small lemon or large plum

Photo Source: What to Expect

Second Trimester

Month 4 (Weeks 14-17)

Around the start of your 4th month of pregnancy, you officially enter the second trimester! Up until this point, your little baby has been pretty transparent. During month 4, the skin starts to get thicker, and hair begins to grow on their body. This includes little eyelashes and eyebrows. The facial features also continue to move so that when they’re born they will be in the proper position on their little head.

Movements become more obvious, and if you have a posterior placenta, you will most likely begin to feel little moments (called quickening) as your baby stretches, brings their hands to their mouth, and turns their head. However, if this is your first baby or if you have an anterior placenta, then it can be a bit longer before you feel anything.

Month 4 is also an exciting time because your baby starts doing things that he or she will do after birth, such as sucking his or her thumb, swallowing, and even smiling. Ears and eyes are more developed now as well, so your baby will start to become more sensitive to lights and sounds. So feel free to sing and talk to your baby at any point now!

Month 4 Overview

• Prints on fingers and toes form
• Vocal cords have formed
• Facial features continue to move to the right position on the face
• Hair starts to grow
• Skin starts to thicken
• Movement increases as limbs and joints continue to develop
• Sucking and swallowing start
• Heart is now pumping lots of blood through the body each day
• Baby begins to react to lights and sounds

Baby’s size at the end of 17 weeks: Onion or turnip

Photo Source: Times of India

Month 5 (Weeks 18-22)

Although some soon-to-be mommas will start to feel movement in month 4, month 5 is when it happens for most! If this is the case for you, you may start to pick up on when your baby is awake and when they are sleeping, as sleep cycles begin to form! 

Other movement you may start to notice at this time is hiccupping. As their lungs continue to develop, their diaphragm may spasm, causing hiccups. The bronchioles within the lungs become much more developed during month 5, which will help your baby to breathe (and cry) after birth.

Most of the organs have developed to a point where they are functioning well. The heart is pumping cells that are formed by the bone marrow, liver and spleen, and the lobes of the brain are continuing to develop.

In month 5, baby will also start to put on weight and fat. Vernix caseosa and lanugo will also begin to cover your baby’s body. These will continue to form in the coming weeks and as your baby enters the third trimester.

Month 5 Overview:

• Sleep and wake cycles start to form
• Hiccupping starts as lungs continue to mature and develop
• Fat is forming
• External genitalia have formed and can now be detected more accurately on an ultrasound
• Brain lobes are developing 
• Vernix Caseosa and lanugo are now starting to cover baby
• Baby may start drinking amniotic fluid to practice swallowing

Baby’s size at the end of 22 weeks: Papaya

Photo Source: What to Expect

Month 6 (Weeks 23-27)

During the last few weeks of your second trimester, your baby will put on increasing weight. By the end of this trimester, the odds of your baby being able to survive if you went into labor early are pretty good with quality neonatal care. The lungs have developed, which, along with the brain, are one of the last things to mature. However, there is still much more growth that needs to happen in the coming months.

In month 6, those little eyelids will finally start to open, and your baby may start to blink. Melanin begins to be produced, which will eventually give rise to your baby’s unique eye and skin tone. The ears will also begin to hear better, which may even result in your baby becoming startled when loud sounds occur.

That lanugo and vernix caseosa that began to form in month 5 will continue to form, and your baby may continue or start to grow hair on their scalp if they haven’t already.

Month 6 Overview:

• Lanugo and vernix caseosa continue to form
• Melanin is produced
• Eyelids may open for the first time
• Hair starts to grow on baby’s head
• Baby continues to gain fat
• Ears develop more fully and can hear sounds even better

Baby’s size at the end of week 27: Cauliflower or cabbage

Photo Source: Twin Stuff

Third Trimester

Month 7 (Weeks 28-31)

You have finally reached your third trimester! For a lot of moms, this is the point where nesting begins, you become increasingly uncomfortable, and you just want to meet your baby already! Luckily, there’s just a few months left and you’ll be there! In month 7, your baby begins prepping for birth just as you are doing! They’re body may start to turn with their head down. 

One of the biggest things that happens this month is the great strides in your baby’s brain development! The bumps of the brain form, which allows for greater surface area. This means more brain cells! As a result of this rapid brain development, your baby will start to be able to regulate his or her own body temperature, something they will need to be able to do once they’re no longer inside of you! This brain development also aids your baby in their ability to respond even better to stimuli like light and sound.

Month 7 Overview:

• Baby starts to turn with head facing down
• Baby blinks more
• REM sleep cycles begin
• Brain develops, bumps in brain form, baby can interpret and respond to stimuli
• Baby continues to swallow amniotic fluid

Baby’s size at the end of 31 weeks: Coconut

Photo Source: BabyCenter

Month 8 (Weeks 32-35)

Things start to wind down in month 8 of pregnancy. And although it’s exciting, it’s important to remember that each and every day your baby is inside of you will give your baby additional time to develop and grow!

As talked about previously, some of the last things to mature are the brain and lungs. In month 8, the brain continues to get larger along with the rest of their body. As your baby puts on weight and strength and as the bones harden, you may get increasingly uncomfortable as they kick and punch your other abdominal organs. However, the bones in the skull will stay soft and moveable. This will allow easier passage during delivery.

Your baby has also put on a lot of fat, and the skin has thickened to the point that you can no longer see through baby’s skin! Your baby is also drinking LOTS of amniotic fluid each day. This will help your baby’s GI system be ready for milk.

Month 8 Overview:

• Brain and lungs continues to grow and develop
• Skeletal bones continue to harden
• Bones in skull stay soft to aid in delivery
• Baby is drinking around a pint of amniotic fluid per day
• Baby is no longer transparent
• Lots of fat continues to build up
• All organs (except lungs) are fully developed

Baby’s size at the end of week 35: Swiss chard or honeydew melon

Photo Credit: SMA Baby

Month 9 (Weeks 36-40)

You’re almost there! Month 9 can feel like it never ends. But just remember that your baby will come when they’re ready. Trust your body and know that it knows when it’s time.

In the final weeks of pregnancy, your baby will start to shed the lanugo, or body hair, that helped protect it in the womb. Although most will be gone by birth, you may notice some little tufts of hair (especially behind the ears and on the arms) at birth. 

Your baby’s lungs also finally reach the point where they can function independently and are ready to breathe air! And of course, that fat and weight continues to be piled on at a rapid pace. The brain is also growing in size (although it is technically fully developed).

Your baby will also drop down into your pelvis to get ready for delivery. You may feel a lot of pressure on your cervix. Just know that it means you will meet your sweet baby soon!

Month 9 Overview:

• Weight is gained as baby grows
• Lungs are fully developed and ready to breathe air
• Lanugo is shed
• Brain continues to grow in size
• Baby drops into pelvis to prepare for delivery

Baby’s size at 40 weeks: Small pumpkin

For more info on your pregnancy, check out of post Your Guide to a Healthy Pregnancy from Conception to Birth, and visit our blog at babycubby.com!

Resources:

Mayo Clinic
Cleveland Clinic
What to Expect
American Pregnancy Association
UI Health

Note: Although this information was checked on multiple sources to check for accuracy, remember, that every baby is different and will develop at slightly different rates. And due to variations on findings in pregnancy studies that have been performed, there will be some slight discrepancies as to exactly when certain things develop in a fetus.