Pregnancy Fetal Development

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The pregnancy fetal development period is referred to as the time between conception and birth when a baby develops and grows inside the mother’s womb. Due to the fact it is impossible to know precisely when conception occurs, a gestational age is calculated from the first day of the mother’s last mental period to the current date.

Pregnancy fetal development is measured in trimesters and weeks. At week 1 and 2 of pregnancy, the woman is obviously not pregnant yet.  This is usually when her body is preparing for the baby. A normal gestation or fetal development last anywhere from 36 to 42 weeks. You can follow your baby’s week by week development in this article. 

First Trimester - Pregnancy Fetal Development

Week 1 to 2  

This is the first week of pregnancy fetal  development. At this point, conception has not taken place yet. The egg will be released from the ovary during the second week. You are most likely to conceive at this point if you have unprotected sex.   

Week 3 

At this point, you baby is a ball of cells called blastocyst. This blastocyst already contains a fullest of DNA from your partner and you. This will determine the baby’s eye color, sex, and other traits. 

Week 4

The blastocyst has officially become an embryo which is about the size of a poppy seed. Over the course of the next 6 weeks, your baby’s organs will begin to develop and some will begin to function. 

Week 5 

At this stage of pregnancy fetal development, your baby’s heart starts to beat- at a faster rate than yours. Also, her entire ‘body’ is just about the size of a sesame seed.  

Week 6

Facial features like nostrils and eyes begin to form and little buds show up where leg and arm will develop. 

Week 8

Legs and arms are growing and your baby now has tiny fingers, as well as upper lip and nose. She starts moving very slowly at this point, but you can’t observe it. She is about 5/8 inch long. 

Week 9 

She begins to develop eyes, though the eyelids are fused shut at this point. She has lost her ‘tail’ and begins to look human at this point.

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Week 10 

The embryo has finally become a fetus and the vital organs like brain, liver, intestine and kidney begin to function. Toenails and fingernails are starting to form. 

Week 11

At this stage, your baby is fully formed. Her bones are undergoing the process of hardening. Her genitals are also developing externally. She can hiccup though you may not be able to feel it at this point. 

Week 12

At this point, you are most likely able to hear your baby’s heartbeat at a prenatal checkup if you have not already heard it at an early ultrasound. Now your baby weighs half an ounce and is about 2 inches long.

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Second Trimester - Pregnancy Fetal Development

Week 14

At this point, your baby’s kidneys are producing urine. And she releases it into the amniotic fluid. She can also make facial expressions and may also have discovered thumb sucking. 

Week 15

You baby can now see lights that filters in from outside your womb, though her eyelids still remain shut. 

Week 16

You can detect the sex of your baby at this point.  This usually happens when you go for your mid-pregnancy ultrasound.  It normally happens around week 16 to 19. 

Week 18 

You will probably feel the movement of you baby in the next couple of weeks if you haven’t yet.  The time of feeling the baby’s movement differs from mother to mother but it is all within week 18 to 22. 

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Week 19 

You baby can now hear your heartbeat and external sounds like you and your partner's voice. Her skin is wrinkly and mostly covered by a protective waxy coating. She weighs about 8 ½ ounces and measures about 6 inches from head to toe. 

Week 23

She has developed a sense of movement at this point and can feel the motion if you dance. Her sense of hearing gets better. You may also feel her squirming inside your belly. 

Week 26

She starts to develop taste buds. Her brain is also growing rapidly. She now weighs over a pound and is almost a foot long. 

Week 27

Your baby starts to develop lungs but the lungs will not be functional for some more weeks. He also starts practicing for life outside be inhaling and exhaling amniotic fluid, opening and closing eyes, sleeping and waking at intervals and sucking her fingers. 

Third Trimester - Pregnancy Fetal Development

Week 28 

Your baby’s eyesight is improving and she now spots eyelashes. She is also capable of dreaming. Billions of neurons are developing in her brain. She is getting bigger and longer. 

Week 32

He has grown cute little toenails and fingernails. You baby has also begun to plump up in preparation for birth. Now she weighs 3 ¾ pounds and about 17 inches long. 

Week 34

You baby central nervous system and lungs continue to develop. Her skin has become smooth and soft and she is filling out and getting rounder. She now weighs 4 ¾ pounds and 18 inches long. 

Week 37 

You baby is now getting ready to be born. She is now considered ‘early term’. Babies delivered at this point usually do well but normally, she will stay in the womb for a few more weeks to give her lungs and brains time to fully mature. 

Week 39 

You baby is fully ready for life out and is now considered full-term. The average length of a newborn is 20 inches, while the weight is 7 ½ pounds. 

Week 41

You have now passed you "due date" and you baby is now considered late-term. She is post-term if you are still pregnant at week 42. At this point, you baby’s health is monitored with tests such as biophysical profile or non-stress test. To avoid any complication, you doctor may talk to you about possibly inducing labor in the next week or two.  If you go over your due date, don't worry too much.  A study conducted at Harvard found that gestation has historically been 41-1/7 weeks long, not the typical 40 weeks we track in most developed countries.  

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