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Fetal Development

The First Trimester

During the first trimester, or the first three months of pregnancy, your baby begins to develop all of its organs and systems. In the first 3 months your baby is very sensitive to the things that your body provides him or her with. Any exposure to teratogens while you are pregnant, and especially during these early months, could cause birth defects in your child. A teratogen is any substance that can cause a birth defect. This may include certain chemicals, medications, infections, or diseases. Birth defects are most common due to exposure during the third to the eighth week of pregnancy. To reduce the risk to your baby and yourself you should avoid harmful substances while you are pregnant. This may mean changing your lifestyle, diet, or environment. Drugs (both legal and illicit), alcohol, tobacco, certain chemicals, and sexually
transmitted diseases can harm you and your growing baby. Check with your doctor to see which prescription and over the counter medications are safe for your growing baby or visit this website to learn more.  You can also call the following:

  • Motherrisk helpline: 1-877-439-2744.

First Month

medically accurate illustration of a pregnant woman - week 5

medically accurate illustration of a pregnant woman – week 5

  • At the end of the first month your baby is about the size of a poppy seed.
  • Your baby does not yet have a human shape. At this stage a human begins to form with what is called a neural tube. This neural tube forms what later becomes the central nervous system of the body. The central nervous system consists of the brain and spinal cord. The neural tube begins to close during the start of the second month and it is very important that the neural tube closes in a proper way. To reduce the risk of this tube not closing your body needs folic acid. If the neural tube does not form properly the baby may be born with a neural tube defect. Neural tube defects can present with many serious and life-long conditions. The most common neural tube defect is spina bifidia: a condition that can cause physical disabilities, development delays, and abnormalities that may require surgery. This is why you need to prenatal vitamins with folic acid while you are pregnant. With the right prenatal vitamins and folic acid supplements you can greatly reduce the risk of neural tube defects in your baby. Visit your doctor or pharmacist to discuss which vitamins and supplements are right for you. (Public health Agency of Canada)

Second Month

medical accurate 3d illustration of a fetus week 6

medical accurate 3d illustration of a fetus week 6

  • At week 5 the baby is about the size of an apple seed.
  • Your baby looks much like a tadpole or a seahorse at this time.
  • The structures for all of your baby’s majour organ systems begin to develop at the start of the second month. These systems include the respiratory system (the lungs and muscles that help us breathe), the cardiovascular system (The heart and blood vessels, or veins and arteries), digestive system (stomach and intestines), and central nervous system (brain and spinal cord).
  • The structures of your baby’s cardiovascular system (heart and blood vessels or veins and arteries) have begun to form. At about 6 weeks your baby’s heart starts to beat.
  • During the second month your baby’s brain is growing quickly. This fast brain growth causes your baby to have a head that is about twice the size of their body.
  • Your baby’s kidneys have begun to form at about week 7.
  • Your baby now has a face and structures for a nose, eyes, and ears.
  • The baby’s arms and legs have begun to grow longer and they now have elbows and wrists.

Third Month

medical accurate 3d illustration of a fetus week 9

medical accurate 3d illustration of a fetus week 9

  • At week 9 the baby is the size of a grape.
  • In the beginning of the third month, around week 10, your baby begins to take on more of a human body shape but the head is still much larger than the rest of the body.
  • Your baby’s arms and legs continue to develop during the third month and by week 10 your baby has fingers and toes. The baby can move his or her arms a little bit at this time but the movements are too small for the mother to feel.
  • By the end of the third month the head growth begins to slow down and the head is more in proportion with the rest of the baby’s body.
  • By the end of the third month the heart is fully developed.

The Second Trimester

Fourth Month

medical accurate 3d illustration of a fetus week 15

3d illustration of a fetus week 15

  • At 4 months, the 14th week, the baby is the size of a pea pod.
  • Your baby’s skin is see through and their body is growing in size very quickly.
  • Your baby’s arms and legs continue to develop and the baby is able to make bigger movements, but they still cannot be felt by the mother. By the end of the fourth month the baby is very coordinated with their movements and he or she is able to touch their own face and toes. The baby now responds to loud noises and unexpected movements with a startle reflex.
  • At the end of week 14 the baby’s external genitalia is well developed and an ultrasound
    can show whether the baby is a boy or girl!

Fifth Month

medical accurate 3d illustration of a fetus week 20

medical accurate 3d illustration of a fetus week 20

 medically accurate illustration of a pregnant woman - week 19

medically accurate illustration of a pregnant woman – week 19

  • At week 18 the baby is the size of an onion.
  • The baby is now covered in a white greasy film called vernix caseosa. This substance protects the baby from the amniotic fluid that surrounds them as it is very drying to their
    skin. The baby will also grow a fine covering of hair over their body called lanugo. This hair helps to hold the greasy film of vernix on the skin.
  • The baby has eyebrows on their face and hair on his or her head.
  • The baby’s bones are starting to grow and harden.
  • The baby has little buds in it’s mouth where teeth will begin to develop.
  • The baby begins to suck and swallow inside of the uterus.
  • The baby looks very much like a human baby at this time but is still not developed well enough to survive outside of your body.

Sixth Month

medical accurate 3d illustration of a fetus week 25

medical accurate 3d illustration of a fetus week 25

  • At 6 months, 22 weeks, the baby is the size of a carrot.
  • During this month the baby begins to put on lots of weight. Some of this weight is a fat called ‘brown fat’, this fat helps baby’s retain heat and stay warm.
  • During the sixth month the baby’s heart has begun to beat strong enough to be heard with a stethoscope by your health care provider.
  • At the beginning of the sixth month the baby’s lungs begin to grow little balloon-like structures called alveoli. These alveoli are surrounded by blood vessels. These structures are what allow us to take oxygen from our lungs and carry it to the rest of our body. They also allow our bodies to take the carbon dioxide that we create in our body and get rid of it when we exhale. In week 26 the lungs begin to produce a substance called surfactant. Surfactant is very important in the lungs because it helps the alveoli to remain inflated. Though many of the other systems are developed in your baby and functioning well, the lungs are still immature. A baby born prematurely any time after 26 weeks may be able to survive with some help to breathe. If your doctor believes you may deliver prematurely at this time they are able to give the baby a needle in utero. This needle is a steroid that will help the lungs to develop quickly and give your baby a better chance of surviving outside of the mother’s womb.
  • The baby now has the strength and ability to grasp with its hands. The baby should be moving around quite a bit inside of your belly. From the sixth month until birth you should keep track of how many times a day your baby makes movement. Each time you feel your baby make movement mark down the time. You should do to this through the day until the baby makes at least 10 movements. You may notice a pattern of movement and rest times during the day. If your baby’s movement patterns drastically change, or you do not feel the baby move at least 10 times in a 24 hour period, contact your health
    care provider.

The Third Trimester

In the third trimester your baby has developed most of it’s organs and systems. The lungs will continue to develop during these months to prepare the baby to breathe on their own. These months really focus on the baby gaining weight and growing in size. The baby is preparing for birth and gaining everything he or she needs to survive outside of your body. Soon it will be time to meet your new little bundle of joy! Continue to count the kick counts and prepare to bring the baby home.

Seventh Month

medical accurate 3d illustration of a fetus week 28

medical accurate 3d illustration of a fetus week 28

  • At 7 months, 27 weeks, the baby is the size of a cauliflower.
  • The baby has become larger and is now filling the uterus with less room to move. As a result you may not feel the baby’s movements as strongly. The baby is now curled up inside of the uterus with its
    legs and arms tucked in.
  • The baby’s lungs are much more developed and are capable of breathing in air. A baby born prematurely at this time would have a much better chance of surviving outside of the mother.
  • The baby’s central nervous system (brain and spinal cord) has matured much further and can now control rhythmic breathing and the baby’s temperature.
  • The baby can open and close its eyes.

Eighth Month

medical accurate 3d illustration of a fetus week 30

medical accurate 3d illustration of a fetus week 30

  • At 8 months, week 31, the baby is the size of an acorn squash.
  • The baby is usually positioned head down at this stage to prepare for birth.
  • At this stage all of the baby’s organs are well developed except for the lungs which are still maturing a little bit.
  • The baby has senses and can respond to pain, light, and sound and he or she is able to taste sweet and sour.
  • Your baby’s digestive system begins to mature and is now more ready to suck, swallow, digest, and absorb the milk that you will feed him/her when they are born.
  • The baby has grown so much that it now begins to fill the uterus and become crowded with his or her arms and legs tucked in. The baby is very active during this time but the movements are less intense as there is less room for the baby to kick and move. The baby may cry, hiccough, or suck his thumb in the uterus.

Ninth Month

medical accurate 3d illustration of a fetus week 37

medical accurate 3d illustration of a fetus week 37

medically accurate illustration of a pregnant woman - week 37

medically accurate illustration of a pregnant woman – week 37

  • At the beginning of 9 months, week 36, the baby is the size of a pineapple.
  • The baby puts on even more weight in the last months of pregnancy, his arms and legs become more plump as he prepares to enter the world.
  • The baby can now grasp with his hands and fingers.
  • The baby’s movements become much less noticeable as he continues to fill up the uterus and becomes more curled up with his arms and legs crossed.
  • Your baby’s digestive system is now finished developing. Your baby’s bowels are filled with a substance called meconium which will be your baby’s first poop. Meconium is a black tarry colour, but this should begin to change to a yellow seedy colour by about the third day after birth.
  • Your baby has small breast buds for both males and females. The male baby now has testes.