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Fetal development: Your First Trimester: Big Changes

Before your pregnancy test even comes back positive, fetal development is already underway. The following week-by-week calendar lays out the changes.

Congratulations – you're pregnant! What next? As you begin to notice changes to your own body, you’ll probably wonder what’s going on inside. How is your baby developing? Are you having a he or a she, and how soon can you tell? Is your baby’s growth on track? And how soon until you hear that precious little heartbeat?

The good news is that we know a lot about fetal development and its progression. To answer your questions, we’ve prepared a week-by-week calendar to help you learn about your baby’s time in the womb.

Week 1: Starting Out

Typically, doctors and health care providers date the start of your pregnancy from the first day of your last period. It may seem strange to use this as a starting point – you’re not even pregnant yet by this calculation! But this gives doctors a solid date for their calculations and places your pregnancy within the 40 weeks from conception to birth.

Week 2: Fertilization

During ovulation, your egg travels down the fallopian tubes, and in pregnancy there it joins with the sperm to create a zygote. A zygote is single-celled and made up of 23 chromosomes from the father and 23 from the mother.
Chromosomes are genetic material that helps determine various characteristics like our gender, our complexion, the color of our eyes and hair, our height, and even facial features.

After fertilization has occurred, the zygote moves down the fallopian tube into the uterus. From the moment fertilization happens, the zygote has been dividing into a cluster of complete cells. There are two groups of cells: an inner group which becomes the embryo and an outer one that develop into nourishing and protective membranes around the embryo.

Week 3: Implanting

As the zygote travels to the uterus, its growth has reached about 500 cells and we now refer to it as a blastocyst. The walls of your uterus is where the blastocyst will settle and begin drawing nourishment. Here it will also begin to form the placenta, which will provide additional nourishment throughout your pregnancy.
It is at this stage that pregnancy tests being turning up positive.

Week 4: Embryonic Development

Week four sees the development of the baby’s vital organs including the brain, spinal cord, and heart. Since your baby is only 1/25 of an inch, these developments can only be seen by microscope.

Three layers of cellular development will separately develop into differing organs of your baby. The ectoderm or the top layer is where the neurological organs of the brain, spinal cord, nerves and backbone will develop. This midline groove down your baby's body is called the neural tube.

The middle layer is where the heart and circulatory system have their beginnings. This is referred to as the mesoderm and provides the tissues that will become your muscles, kidneys, the reproductive system as well as your baby's bones.
Lastly, the innermost layer— the endoderm — is where your baby’s lungs, intestines and bladder will grow along a rather simple mucous membrane lined tube.

Week 5: The Heart Beats

3 Weeks Since Conception (Week 5)

If you have a pen, notice the tip. That is the size of your baby by the fifth week. Only 1/17 of an inch long.

But big things are going on in this week. Blood vessels are completing circuits as the circulatory system, driven by your baby’s newly developed heart, is taking form. At this stage, even though you can’t hear it yourself, an ultrasound exam can pick up the familiar rhythms.

Blood is often called the river of life and with good reason. As this stage progresses, the circulatory system becomes your baby’s very first fully functioning organ system.

Week 6: The Neural Tube Closes

4 Weeks Since Conception (Week 6)

From 1/17 of an inch to 1/8, your baby has doubled in size during this rapid growth week. With the closing of the neural tube along your baby's back the heartbeat has now assumed a natural, regular rhythm.

We also begin to see during this week the appearance of basic facial features. The secondary organ systems begin to develop, the opening of the mouth and what will become the ear canals lead to the formation of the digestive and respiratory systems.

Connective tissue too has become to form along the midline of your baby, miniscule bits of tissue that will eventually become ribs and muscles  as well as tiny buds which will soon be recognizable as arms and legs.

Week 7: The Umbilical Cord

5 Weeks Since Conception (Week 7)

The tip of the pen sized embryo is now just about the size of a pencil eraser’s top. At 1/3 of an inch, your seven week old baby is weighs no more than a single aspirin tablet.

Last week’s beginning of a digestive system is now made visible with the appearance of the umbilical cord linking your baby to the placenta. Also visible, due to the transparency of your baby’s skull are the newly formed cavities and passages that will circulate spinal fluid in your baby's brain.

Becoming more noticeably baby-like, the little arm bud of last week is now paddle waving in the uterus. This tiny arm is joined by more facial definition as a mouth perforation, ear indentations and tiny nostrils are now visible.

Week 8: Toes and Fingers

Webbed fingers and toes have arrived. Your baby’s arms and legs are beginning to define themselves clearly now in this eighth week of pregnancy. Just over ½ an inch, your baby is also forming wrists, elbows and ankles. And just as that takes place, eyelids are forming as well.

Other facial features are also more recognizable at this stage including ears, upper lip and nose’s tip. While 150 beats per minute may sound fast at around two times an adult’s heart rate, this is perfectly on target as your baby's heart becomes more completely developed.

Week 9: Did You Feel That? Movement.

7 Weeks Since Conception (Week 9)

You can’t feel it yet, but those newly formed arms and legs might just very well start your baby moving during this week. Almost a full inch long and about 1/8 an ounce, your baby is looking every day more and more like a little person. This is helped by the shrinking of the embryonic tail we all start with at the base of the spinal cord.

The growth of your baby's head, now almost half the size of his or her body, has been impressive in this time, though babies keep them tucked down close to the chest. This week also sees the beginning of nipples and hair follicles. Internally, reproductive organs, either testes or ovaries, are forming as do other internal organs including the pancreas, bile ducts, gallbladder and anus.

Week 10: Multiplying Neurons

8 Weeks Since Conception (Week 10)

That embryonic tail that started shrinking in the last week will have disappeared totally by week ten and your baby's vital organs are well established. The tiny webbed fingers and toes are now completely separated as your baby’s skeletal bones are now beginning to form. Along with bones, tooth buds are starting to take shape.

But the biggest news is the production of neurons in your baby’s brain. In this week alone, every single minute of the day, your baby's brain will produce 250,000 new neurons.

At this point, if you are having a boy, your baby’s testes should begin production of testosterone, the male hormone. Likewise, at this stage, your baby's eyelids have ceased to be transparent and the shape of the outer ears are finalizing.

Week 11: Are You Having a Boy or a Girl?

9 Weeks Since Conception (Week 11)

It’s now official – your embryo is a fetus, but from here on out the changes will be even more dramatic. For the next ten weeks, or the halfway point of your pregnancy, your baby will grow three times as long and will increase his or her weight by 30 times what it is now.

The placental blood vessels are multiplying rapidly and increasing in size to feed this growth. This week sees your baby's ears moving upwards and along the side of the head. Most importantly to many parents, by this week’s end, you should be able to recognize your baby's external genitalia.

Week 12: Fingernails and Toenails

Almost one full ounce – that’s how much your baby weighs at this point. While your baby’s whole body only measures 3 inches long almost half of that is the baby's head.

And on that face, we begin to see a much more defined chin and nose. The ends of the feet and hands see big developments this week as well, as tiny fingernails and tonenails will have arrived.

Caring for Your Baby

For the sake of healthy baby development, it is important for the parents to make healthy lifestyle choices. This is true even prior to conception. The following short list of do's and don'ts are important to remember:
·     Every day, take your prenatal vitamin
·     Regulate your weight to a healthy level
·     After doctor approval, be sure to exercise regularly
·     Maintain a healthy diet
·     Avoid stress and take care of any chronic health conditions
·     Attend regular prenatal checkups
·     Inform your doctor if you are taking any medications


·     Smoke
·     Drink alcohol
·     Do illegal drugs

Your baby deserves the very best start and that begins with you. Every day brings a new change so be sure to take good care of yourself and you will take good care of your baby.


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