Fetal Development Week by Week

Pregnancy marks an exciting time for parents. The thrill of an impending new arrival to the family drives most parents to the internet and parenting magazines and books, to find out more about how their baby is developing. Every parent wants to do whatever they can, to provide the best possible conditions for the growth and development of their little one — and this starts even before birth.

The 40 weeks of pregnancy are usually divided into three “trimesters”, with each trimester lasting about 3 months.

The first trimester: Months 1 – 3 or week 1 through week 14

The second trimester: Months 4 – 6 or week 14 through week 28

The third trimester: Months 7 – 9 or week 28 through week 40

Let’s take a closer look at the growing baby, week by week.

Pregnancy is “officially” for 10 months, or 40 weeks, because it includes the first 2 weeks (since the first day of the last period), when a woman isn’t really pregnant. [ACOG Prenatal development]

We count 40 weeks of pregnancy from the first day of a woman’s last period. So during week 1 and 2, a woman isn’t pregnant as yet, but this is the time when the body is preparing to release the egg, which happens in the middle of her cycle (roughly two weeks after the first day of her last period).

The egg is now released from the ovary and moves through the fallopian tubes to the uterus (womb).

If a man and woman have intercourse around this time, the sperm are released into the female reproductive tract. Some of these sperm may reach the womb and enter the fallopian tubes.


Conception takes place if the egg encounters a male sperm which successfully penetrates and fertilizes the egg. In this case, the fertilized egg, now called a “zygote”, now starts multiplying to form a cluster of cells by the time it reaches the womb.

This cluster of cells (called morula) is roughly the same size as the zygote. Now the structure changes, so that it encloses some fluid within, thus forming a cluster of cells called a blastocyst. This blastocyst is made up of the inner group of cells which becomes the embryo, and the outer group of cells which will help to protect and nourish the embryo. The embryo will eventually grow and develop into the baby.

After floating in the uterus for a few days, the blastocyst attaches and burrows into the wall of the womb. This process is called implantation.

Now the embryo begins to grow in the lining of the womb. There are three distinct layers, each of which gives rise to different parts of the body.

  • The innermost layer, endoderm will develop into bladder, gut, stomach and lungs (i.e. the digestive and respiratory system)
  • The middle layer, mesoderm, will develop into the blood vessels, heart, bones and muscles.
  • The outer layer, ectoderm will develop into the nervous system, brain, skin, teeth enamel, eye parts and nails.

At this time, since the placenta is not yet fully formed, the embryo gets it’s nourishment from a “yolk sac”.

Surrounding the embryo is the fluid-filled amniotic sac.

Week 5 is when the "embryonic period" begins. This is when all of the baby’s important structures and organ systems will develop. Around this time, a woman misses her period and will perhaps begin to suspect that she is pregnant.

This week, the baby’s nervous system is developing. In the “ectoderm” (see Week 4), a layer of cells will fold to form the hollow “neural tube” that becomes the spinal cord and brain.

Also, the heart is beginning to form as a tube-like structure for now. Blood vessels start to develop, some of which will become the umbilical cord to connect the baby to the mother.

The embryo is curved and is covered with thin, almost transparent skin. While it’s too early to see signs of a “baby bump” in the mum’s belly, around this time, your baby will develop a “bump” at the head side of the neural tube (see Week 5), and where the heart is supposed to be. Both these bumps will develop into the brain/head, and the heart.

Arms and legs start to grow and look like little limb buds, and there are small dimples and thickenings in the head which will soon grow to become the ears and eyes.

Because the brain is growing so fast, the head also grows faster than other parts of the body and has a large forehead. The eyes and inner ears develop slowly, though it will be some time before the outer ear is seen.

The limb buds now start growing cartilage, which will eventually become the bones. The arm buds also grow and the ends start to become flat – these ends will become the baby’s hands.

The nervous system continues to develop.

Picking out names for the baby is fun, though that usually happens later in the pregnancy. However, this week, your baby gets to change names — from embryo to a “foetus” now.

Your baby is still getting nourishment from the yolk sac, as the placenta continues to develop and form structures called “chorionic vili” which will be important to help nutrients reach the baby, and waste products be transferred to the mother.


The legs continue to become longer and develop cartilage.

The end of week 8 usually means it’s the second period that a mother has missed.

This is the week when the baby’s face continues to form. Eye structures become a little obvious; the tongue in the mouth has tiny taste buds. There are small ridges where fingers and toes will later develop.

The kidneys, brain, heart, gut and lungs continue to develop this week.

The baby’s face now has two tiny nostrils developing, along with the upper lip and the jaw bones (imagine this: the jawbones come with all the milk teeth already present).

The ears and ear canals are continuing to develop.

And big news! The heart is now fully formed and beats almost 180 times a minute, which is about twice or thrice as fast as your own heart.

The bones in the face are now formed, ears are a little easier to identify, though the lids of the eyes stay closed. The face and head now make up for a third of the total length of the baby. But the body has started to catch up and become a little more straight instead of curved.

Fingers and toes slowly start to separate and fingernails become evident.

Another milestone! By the time you’ve finished this week, your baby is formed – with all the limbs, bones, muscles and organs (including the sex organs) in place. The skeleton is made of cartilage now, which will start developing into hard bone. From now, your baby has to keep maturing and growing.


Although your baby moves a lot, it’s still too early for you to feel the movements.

Although your baby’s sexual organs (testes or ovaries) are already formed inside the body, the external (outside the body) reproductive organs now start forming. It is still too early for a doctor to know the sex of the child (but remember, though prenatal sex determination is permitted in some other countries, it is not allowed in India).

Around 12 weeks, or a little later, the placenta is fully formed. The placenta helps in the transfer of oxygen and nutrients to the baby, and the removal of waste products to the mother. Both the mother’s and the baby’s blood vessels are seen in the placenta, but the blood does not mix. The placenta is connected to the baby via the umbilical cord.

Around this week, a baby slowly starts swallowing little amounts of amniotic fluid. The fluid enters the stomach, and later the kidneys, which work to push the liquid back into the surrounding, as urine.

Another big step this week: your baby starts to hear sounds – soft sounds from the environment, your (the mom’s) heartbeat, digestive system sounds and voice. Also, even though the eyelids are still closed, the eyes start getting sensitive to light, even to very bright light that you may see.

This week, baby can start making facial expressions, though it’s too early for the baby to control these expressions. The muscles in the limbs start to move, as the nervous system develops. This means that the baby can reach and hold both hands together, and form a fist.


During this month,

On the face, eyelashes and eyebrows start growing. The eyes can move, though the lids are still closed. The baby can open the mouth.

On the hands, fingernails continue to grow and there is now a fingerprint. Babies develop a firm grip

The baby does not have a lot of body fat yet, so the skin appears wrinkled. The baby will put on weight throughout, and especially towards the end of your pregnancy. There is fine hair (called “lanugo”), all over the body, and a white, greasy coating (called “vernix”) to possibly protect the skin in the amniotic fluid.

The baby goes to sleep and wakes up regularly. Because the baby is more active, the mother may be able to feel movement around 17-18 weeks; first-time mothers may be able to discern it between 18-20 weeks for the first time (initially, it may feel like bubbling, fluttering or even a little indigestion).

Also, if the baby is a girl, she already has eggs in her ovaries (think about it – this means that the egg needed to make you was possibly already existing when your mother was around 20 weeks old in your grandmother’s tummy!).


Although the placenta still supplies oxygen from you to the baby, your baby’s lungs begin to practice how to breathe – think of it as a “dress rehearsal” for after birth.

The fine, soft hair (lanugo) that covers the baby’s body may help to maintain the right body temperature. By the time the baby is born, the lanugo disappears.


While your baby has slowly developed a regular pattern of sleeping and waking up, it may not be your schedule (which could mean disturbed sleep at night!).

Sometimes, babies are born before 24 weeks. For these babies, survival is harder since most of the important organs have not developed enough. However, after the 24 weeks are over, babies can be given care in the neonatal care units, though survival is still difficult with higher risks of the child developing a disability.

You will begin to experience and enjoy (well, most of the time at least!) your baby’s movements a lot more from now.

Your baby will move a lot, and can respond to sound and to touch. If there’s a loud sound you hear, your baby may react to it by jumping or kicking, and you’ll be able to feel it. Your baby may occasionally get hiccups – you’ll be able to feel that too.


The baby is also passing urine regularly into the amniotic fluid.

Finally, the eyelids open around this time and your baby will learn to blink. But it’s too early for eye colour to be determined – this is fixed a few weeks after birth.

The heartbeat slows down a little to about 140 beats per minute (though this is still faster than your heartbeat). The lungs, brain, and digestive system are not mature, but they are formed.

The skin gets less wrinkled, as the baby puts on more weight. The vernix and lanugo (see Week 20) now start disappearing.

The lungs are continuing to develop, though it is only around 36 weeks when the baby can breathe properly. The eyes can focus. The amount of amniotic fluid surrounding your baby increases.


You will be able to feel much of your baby’s movements, and every baby will do things a little differently. Only if there is a change in the pattern of these movements should you tell your doctor.

By the end of week 32, the baby is getting ready for the “grand entrance” at birth and is positioned with its head downwards. Sometimes, babies may take a few more weeks to move to this position, so it’s not cause for concern if it hasn’t happened yet.

By the time it’s 33 weeks, the brain and nervous system are developed, and the skeleton is hardening (apart from the bones in the skull which stay soft till after birth).

Your baby has grown so big that there’s not too much space to move anymore, so the baby is curled up, legs bent near the chest. The baby still moves to change position and you will be able to feel this, and even see this.


By the end of the 36 weeks, the lungs are now ready to function, the digestive system is ready to handle breast milk and the baby will be able to suckle for a feed.

While your baby has been getting ready for birth, your uterus is also getting ready —in order to practice for labour, the uterus begins practice contractions or tightenings. These are called “Braxton Hicks” contractions, which are normal but are not yet labour.

The baby’s digestive system now contains what’s called “meconium” – this is sticky, green poo that is usually seen in the initial bowel movements (it may also contain some fine hair – lanugo). Sometimes, a baby may have a bowel movement during labour during labour which may mean that the baby is stressed.

Also, the baby’s head will move into your (the mother’s) pelvis, which makes it seem that the bump has moved lower. When this happens, the head is said to be “engaged”. Sometimes, this only happens when labour begins.

Your own hormones may cause the genitals of your baby to swell a little. But this is normal, and it will reduce soon after birth.

Experts call the pregnancy “full term” from 39 through 40 weeks of pregnancy. You could go into labour any time now, and for most women, it is between 38 and 42 weeks.

Make sure your hospital bag is ready (see article ___) and read more about the basics of labour and delivery (see article ) as you get ready for D-day!

The changes that occur during the embryo and foetal stages of growth and development

(This table can be a permanent feature in the article, across the weeks)




1–4 weeks

0.6 cm

Weight is minimal

5–8 weeks

3 cm (1.25 in.)

1 g

9–12 weeks

7.5 cm (3 in.)

30 g

13–16 weeks

18 cm (6.5–7 in.)

100 g

17–20 weeks

25–30 cm (10–12 in.)

200–450 g

21–25 weeks

27–35 cm (11–14 in.)

550–800 g

26–29 weeks

32–42 cm (13–17 in.)

1110–1350 g

30–34 weeks

41–45 cm (16.5–18 in.)

2000–2300 g

35–38 weeks

50 cm (20 in.)

3200–3400 g


Note: The information mentioned here is meant to be an overall description of the growing baby. Every baby may grow and develop in a slightly different way.

  1. American College of Obstetricians and Gynecologists. Prenatal Development: How Your Baby Grows During Pregnancy . https://www.acog.org/-/media/For-Patients/faq156.pdf?dmc=1&ts=20160426T1000238421.
  2. ACOG Committee Opinion No 579: Definition of term pregnancy. Obstet Gynecol. 2013 Nov;122(5):1139-40. Freudenrich C, Tortora GJ. Visualizing Anatomy and Physiology.
  3. NHS Choices. You and your baby at 0-8 weeks pregnant. http://www.nhs.uk/Conditions/pregnancy-and-baby/Pages/pregnancy-weeks-4-5-6-7-8.aspx.
  4. NHS Choices. You and your baby at 9-12 weeks pregnant. http://www.nhs.uk/Conditions/pregnancy-and-baby/pages/pregnancy-weeks-9-10-11-12.aspx.
  5. NHS Choices. You and your baby at 13-16 weeks pregnant. http://www.nhs.uk/Conditions/pregnancy-and-baby/pages/pregnancy-weeks-13-14-15-16.aspx.
  6. NHS Choices. You and your baby at 17- 20 weeks pregnant http://www.nhs.uk/Conditions/pregnancy-and-baby/pages/pregnancy-weeks-17-18-19-20.aspx.
  7. NHS Choices. You and your baby at 21-24 weeks pregnant. http://www.nhs.uk/Conditions/pregnancy-and-baby/pages/pregnancy-weeks-21-22-23-24.aspx.
  8. NHS Choices. You and your baby at 25-28 weeks pregnant. http://www.nhs.uk/Conditions/pregnancy-and-baby/pages/pregnancy-weeks-25-26-27-28.aspx.
  9. NHS Choices. You and your baby at 29-32 weeks pregnant. http://www.nhs.uk/Conditions/pregnancy-and-baby/pages/pregnancy-weeks-29-30-31-32.aspx.
  10. NHS Choices. You and your baby at 33-36 weeks pregnant. http://www.nhs.uk/Conditions/pregnancy-and-baby/pages/pregnancy-weeks-33-34-35-36.aspx.
  11. NHS Choices. You and your baby at 37-40 weeks pregnant. http://www.nhs.uk/Conditions/pregnancy-and-baby/pages/pregnancy-weeks-37-38-39-40.aspx.
  12. NIH. U.S. National Library of Medicine. Fetal development. https://www.nlm.nih.gov/medlineplus/ency/article/002398.htm.
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