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Your baby’s development month by month

By Kezi Levin May 24, 2021 0 comments

From a tiny ball of cells your baby will go through incredible developmental changes over the forty weeks of pregnancy. Take a look at how your baby will develop from month to month.

The first month of pregnancy

Once your egg has become fertilized, it moves down the fallopian tube and will begin to divide into more and more cells. The egg will reach the uterus three to four days after it has been fertilized.

The cells continue to develop into a ball which takes another two to three days. Your pregnancy begins when this ball of cells attaches to the lining of your uterus. This stage is called implantation.

The second month of pregnancy

At five to six weeks, the ball of cells becomes an embryo and measures 4 - 5 mm. At this stage, a very basic circulatory system and beating heart are developing. The buds for arms and legs are starting to form, as is the umbilical cord. The neural tube is also developing, which will become your baby’s brain, spinal cord and major nerves.

 At seven to eight weeks, the embryo is now 7 – 14 mm in length. Incredibly the heart is now formed, and webbed fingers and toes are developing. The arms that have formed can now bend at the elbows, and the ears, eyes, lips, and liver are starting to form.  The heart is now beating with a regular rhythm,  if you have an ultrasound at some point over the next few weeks it will probably be visible as a tiny, pulsing dot.

The third month of pregnancy

At nine to ten weeks, the embryo develops into a fetus and measures 21 – 40 mm in length. The umbilical cord connects the fetus to the placenta. The placenta is attached to the uterus wall, where it will absorb nutrients from your bloodstream. The umbilical cord carries the nutrients and oxygen the fetus needs and removes waste.

At eleven to twelve weeks, the fetus will now be measured from the crown of their head to their bottom; this is known as the crown-rump length (CRL). The fetus will measure around 6 – 7.5 cm at this stage. The fetus’s toes are no longer webbed, the bones are starting to harden, and the skin and fingernails begin to grow. At this stage, the external sex organs will appear.

The fourth month of pregnancy

This is the start of your second trimester. At thirteen to fourteen weeks, the fetus has a CRL of 8cm, and their hair is starting to grow. At this stage, you may be able to see the biological sex of the fetus on an ultrasound.

At fifteen to sixteen weeks, the fetus measures 12cm. In biologically female fetuses, hundreds of thousands of eggs are forming in the ovaries.

 The fifth month of pregnancy

At seventeen to eighteen weeks, the fetus will measure around 14 – 15 cm. By nineteen weeks, it will measure 16cm, and fine, downy hair will form as well as a greasy layer to protect the fetus's skin. The bones and nerves in the ears now developed enough to function, and can hear all sorts of sounds -- including your heartbeat.

The sixth month of pregnancy

At twenty-one to twenty-two weeks, the fetus will measure 18 – 19cm in length. At this stage, the bone marrow will start to make blood cells, and the fetus’s taste buds start to form.

By twenty-four weeks, the fetus will measure 20cm, and their eyebrows and eyelashes will start to form. Billions of brain cells will develop in your baby's brain over the next couple of weeks. These will control all of your baby's movements and sensory functions.

The seventh month of pregnancy

At twenty-five to twenty-six weeks, the fetus will measure around 23cm. The fetus will begin to develop more fat as the pregnancy continues. By twenty-eight weeks, the fetus will measure approximately 25cm.

The eighth month of pregnancy

You are now in your third trimester. In weeks twenty-nine to thirty, the fetus will measure 29cm. In biologically male fetuses, the testes will begin to descend, this process is usually complete by forty weeks.

By thirty-two weeks, the fetus will measure 28cm, and at this stage, the soft, downy hair that covered the fetus’s body will begin to fall off.

The ninth month of pregnancy

By thirty-four weeks, the fetus is 30cm in length. The fetus’s eyes are now developed, so the pupils will dilate and constrict when exposed to light. The downy hair has nearly completely all gone.

At thirty-six weeks, the fetus measures around 32cm. The skin is now no longer wrinkled as they have gained enough fat and is getting ready for the final stages of development.

The tenth month of pregnancy

At thirty-nine to forty weeks, you will be ready to give birth to your baby. They will weigh on average 7 – 8lbs and be around 46 – 56 cm long.

You may notice that your baby moves much less due to there not being much space. At this point, your baby’s position may have changed to prepare for birth. Ideally, your baby is head down in your uterus.

 Your baby is ready to meet the world.

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