Timeline of Fetal Development – Week by Week

This timeline of fetal development will give you a good idea of how your baby is progressing throughout the pregnancy. It outlines week by week fetal development for your easy reference.

First of all, congratulations on your pregnancy. Whether you are a surrogate mother or an intended parent, pregnancy is sure to be a time of excitement and anticipation. It can also feel a bit overwhelming with the multitude of information available both online and offline. Where do you even start?

Hopefully we can help you to avoid some stress by providing you with this simple timeline of fetal development. This guide to week by week fetal development is designed to make it easy for you to see what you should expect throughout the course of this pregnancy.

It will discuss the myriad of changes that are taking place with your growing child. It will provide a description of the baby’s week by week fetal development as well as give you helpful tips on how to ensure that the baby and the surrogate mother remain healthy throughout the pregnancy.

The following timeline of fetal development should be used as a general guide for a healthy pregnancy. Let it be noted that week by week fetal development may vary due to certain factors, including but not limited to the mother’s health or a miscalculation in ovulation. Gestational age is defined as the age of the pregnancy based on the last normal menses. Fetal age is defined as the actual age of the growing fetus. For the purposes of this timeline of fetal development, we will refer to fetal age.

As far as the measurements are concerned, up to week 20, they will be given in total length from head to buttocks (crown to rump- CRL). After the 20th week, measurements are given as head to heel. Again, these are general guidelines as weight and length measurements do vary from one pregnancy to another.

Pregnancy is broken down into weeks, with a full term pregnancy being considered one that lasts at least 38 weeks. Babies born before then are considered premature or preterm. The length of pregnancy is 266 days from the moment of fertilization. However, it is often difficult to calculate exactly when fertilization occurred, so day one of pregnancy is considered to be the first day of your last menstrual period (LMP). Someone with an average 28 day menstrual cycle would count fertilization as having taken place on day 14 of their pregnancy since ovulation usually takes place 14 days before your cycle begins.

We have divided this timeline of fetal development into trimesters. The first trimester is from the 1st week to the end of the 12th week. The second trimester is from the 13th week to the end of the 26th week. The third trimester is from the 27th week to the end of the pregnancy.

The First Trimester

Weeks 1-4

This period includes the time when you experiences your last menstrual period. So, even though conception has not occurred during this first week, it is still included because your expected date of delivery (EDD) is calculated based on the first day of your last menstrual period (LMP).

This is the period when ovulation, fertilization, and conception occur. Implantation usually occurs about 10-14 days after conception. Did you know that your baby’s gender is determined upon fertilization? However you won’t be able to find out whether you will have a boy or a girl until the second trimester. It is also at this very early stage that your baby begins to develop neural tubes, which are the precursors for the nervous system and serves as the foundation for your baby to think, feel, sense, and more. Pretty amazing stuff, right?

Weeks 5-6

Right now your baby’s heart begins to beat and you might be able to catch the amazing sound on the screen at an ultrasound. The umbilical also develops at this point, enabling your child to get the nutrients he or she needs in addition to performing functions such as pumping oxygen and removing waste.

The four chambers of your baby’s heart are now functioning and other organs, such as the lungs and the brain make their appearance. This is also the period when the arms and legs begin to develop. During this period, the eye lenses, nostrils, intestines, and pancreas also being to develop.

Weeks 7-8

At this point in the timeline of fetal development your baby is developing a lot of organs and body parts. Fingers and feet begin to develop and little nubs that will become toes pop up. Your baby begins to form eyes, ears, and a nose, although they still look more like an alien than anything else right now.

He or she will begin to develop intestines (oddly enough, outside of the body) as well as teeth (bet you didn’t know that, did you?) During the 8th week, your baby will begin to form cartilage and bones, his or her eye structure will become more normal. The intestines will move from their position in the umbilical cord and into the abdomen. His or her fingers and toes will also make an appearance, though they will be short and web-like right now. Week 8 is when your baby will begin growing – at this point your baby will be .61 inches long (from crown to rump) and will weigh .04 ounces.

Weeks 9-10

At this point your baby has started to move around in the womb, although it will not be strong enough for the movement to be felt. He or she is able to bend and flex as well since the joints are now developed. He or she even has fingerprints! By this time your baby can grab at things with his or her fingers.

This phase marks the end of the most critical part of your baby’s fetal development and is set to develop quickly. Week by week, fetal development will grow in leaps and bounds. It is now that the placenta begins to function, removing waste and providing nutrients. An ultrasound at this point will reveal a baby whose head is half of its length (1.22 inches) and who weighs about 0.14 ounces.

Weeks 11-12

Right now nearly all of your child’s organs and body parts are fully formed and beginning to function, including the liver and the pancreas. Their fingers and toes are no longer webbed and he or she is beginning to grow hair and nails. The genitals will also begin to develop and in a few weeks you might be able to get a peek at whether you will have a boy or girl.

Amniotic fluid, which serves to cushion your baby, will develop along with the pancreas. Your baby’s vocal cords also begin developing. Your baby is now 2.13 inches long and weighs .49 ounces.


The Second Trimester

Weeks 13-14

At this point in the week by week fetal development timeline, your baby is practicing for when he or she will be able to breathe; the eyes and ears are still developing; and you will find that he or she is doing more and more with their hands. Your baby will also begin to develop a layer of fine hair on his or her face which will eventually spread to the entire body.

This is called lanugo and will be shed right before you deliver. It may sound strange and maybe even alarming, but it is there to protect your child’s skin as he or she floats around in liquid in the womb. He or she is now approximately 3.42 inches long and weighs 1.52 ounces.

Weeks 15-16

At this point in the timeline of fetal development, your baby’s genitals will be fully developed by now, and you just might be able to get a peek on an ultrasound. His or her skeletal system continues to develop, as does the mouth. Actually, the baby may now be able to actually taste what the mother ingests. He or she can now breathe ad will inhale and exhale small amounts of amniotic fluid, building up the lungs. Your baby will also be practicing facial gestures as well as reflexes such as sucking, blinking, and swallowing. Even more excitedly, fluttering movements may be felt now as the baby moves around. At 4.57 inches, your baby’s body is now longer than its head. He or she now weighs 3.53 ounces.

Weeks 17-18

Your baby recycles amniotic fluid by swallowing it and is accumulating meconium (which will be baby’s first stools) in the bowel. He or she is beginning to develop fat beneath the skin that will serve to provide both energy and warmth. The skeletal system is shifting from cartilage to flexible bone. Though the eyes are still sealed shut, they are pointing forward now. In addition to lanugo, your baby is now covered in a white, cheese like material called vernix which helps to further protect the skin. Your baby can now hear things (such as your heartbeat) and may actually become startled at loud noises. As air sacs called alveoli form in the lungs, your baby now may go through the movements of crying, but it does not make a sound. You may also be able to notice that he or she has a pattern of sleep and wakefulness, much like a newborn as well as that he or she has a favorite sleeping position. He or she is now 6 inches long and weighs approximately 7 ounces.

Weeks 19-20

If you are having a girl, her uterus is beginning to develop and she will have about 6 million eggs – an amount that will decrease to 1 million by the time she is born. If it is a boy, his genitals will be distinct and recognizable. As your baby’s legs begin to grow, sensations will shift from flutters to full out kicks. Underneath the layer of vernix, your baby’s skin is thickening and developing. His or her nails and hair are also growing. The baby continues to grow, taking up more and more space in the uterus. He or she is now 6.3 inches long and weighs 11 ounces.

Weeks 21-22

Your baby is now producing blood cells and your baby and the placenta provides all of the nutrition your baby need. The baby can now hear and recognize the mother’s voice and may even respond when music is being played. Now is a good time to talk, read, and sing to the baby. The baby’s eyebrows and eyelids are now fully developed and he or she has fingernails. Your baby can now also experience touch and will often explore this sense by stroking its face, sucking its thumb, and moving body parts around. The reproductive is also continuing to develop as boys’ testes descend from the abdomen and girls have fully developed uterus, ovaries, and vaginas. Your baby now weighs 11 inches (crown to heel) and weighs a whopping one pound.

Weeks 23-24

Your now “viable” baby is really active now as he or she seems to do gymnastics in the womb. He or she is now proportioned like a thin newborn (it still has not developed that adorable baby fat). The skin pigment is now forming and soon sweat glands will begin forming under the skin. If born now, your baby would have a 20% chance of survival under medical care but will likely have some health issues. He or she is now 11.8 inches long and weighs 1.3 pounds.

Weeks 25-26

If born now, your baby has a 50% chance of survival. The spine is beginning to form in order to support the baby’s growing body and the brain is growing rapidly. His or her hands are fully developed and dexterity is constantly improving – he or she can now make a fist and grab things. In fact, exploring its body and its environment is your baby’s favorite pastime. He or she is now 14 inches long and weighs 1.7 pounds.


The Third Trimester

Weeks 27-28

You guys have made it to the third trimester in the timeline of fetal development. Congratulations! Your baby now looks much like he or she will at birth. Your baby is gaining in so many ways. Muscle tone is improving, body fat is developing, and the brain is growing at a rapid pace. Your baby can now also open his or her eyes and may even respond to a flashlight being shined upon the belly. Rapid Eye Movement also indicates that your baby may be dreaming. He or she is about 14.8 inches long and weighs 2.2 pounds. If born now, survival is 80% likely with chances for serious complications.

Weeks 29-30

Your baby may now show preferences (as well as dislikes) for certain flavors. His or head grows to accommodate its rapidly developing brain. The baby is also growing in leaps and bounds and its living quarters are becoming more and more cramped in its week by week fetal development. However, this doesn’t keep the baby from moving around, kicking, and stretching. The baby continues to store fat and put on weight to prepare for life outside the womb. In fact, he or she will gain about one pound each week up until the 38the week. Right now, he or she clocks in at 3 pounds and is 16 inches long.

Weeks 31-32

Your baby’s major organs are now functioning and will continue to develop. There may be a decrease in fetal movement as the sleeps for most of the day and as space to move about in becomes less available. From this point in the timeline of fetal development, weight gain will come more rapidly than length growth. In fact, between now and birth, your baby will double its weight. Right now, he or she is 17 inches long and weighs about 4 pounds.

Weeks 33-34

Your baby is continuing to recycle amniotic fluid – they drink about a pint of it each day (and urinate about the same amount!) The lanugo begins to fade as your baby’s real hair begins to grow. His or her nails are now so long that they could actually scratch their face. Your baby may have turned into the head down, birthing position by this point in the timeline of fetal development. It is becoming plumper and plumper as it accumulates more fat in the arms and legs. Its immune system also gets a boost in the form of antibodies that cross the placenta. Your baby is now 18 inches long and clocks in at about 5.25 pounds.

Weeks 35-36

Your baby’s lungs are now just about fully developed. In fact, he or she is almost ready. The only things left to develop are the lungs. He or she will have coordinated reflexes, responding to noises, light, and even touch. They should be moving around every day though the cramped space means that movement may not be as forceful as before. Your baby may soon drop into the birth canal, though this may not occur until right before labor. Most babies will be in the birth position by now and any movement will likely be from side to side. Right now your baby is about 18.5 inches and 6 pounds.

Weeks 37-38

At this point in the timeline of fetal development, if your baby is born he or she will be considered full term. In fact, development is pretty much done and the baby’s only job now is to gain weight and pack on the fat. The baby will continue to gain about one ounce each day from this point on. Right now, he or she is about 20 inches long and 7.5 pounds.

Weeks 39-birth

Congratulations! You made it! Your baby is now preparing for birth. He or she is likely in a head down position against the birth canal. Movement will still be present, but reduced because there is simply no more room to move around in. Your baby is ready for life outside the womb! He or she will be between 6 to 9 pounds and approximately 20 inches long.

What would you say is the easiest trimester?

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