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Fetal Development: Stages of Growth  

 

The First Trimester   

From Fertilization  It all starts after fertilization Fetilization typically happens 2 weeks after a woman's last menstrual period (LMP).  By 4 weeks the brain and spine have formed.  By 4 to 5 weeks the heart has formed By 6 weeks a heart beat and the embryo may be seen by internal scan. The yolk sac is seen, the structure that feeds the baby initially.  

 
 
 
By 8 weeks the limb buds are forming. The embryo is the size of a bean. The the eyes, nostrils and rest of the face, fingers and toes are developing and by 10 weeks all the major organs have formed. Fetal movement is now seen on ultrasound scan. 
During development, the intestines start on the outside of the fetus (physiological gut herniation) and start to move inside the abdomen. This process is completed by 12 weeks.  
There is more growth and by 14 weeks, the fetus measures 8.5 cm. By 13 weeks, the fetus is 6 to 7 cm long and weighs 73 grams . The placenta is now well grown and has taken over from the yolk sac.  
 
 
 
 
 
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The Second Trimester  

From 14 weeks 
 
At 14 weeks, the fetus measures 8.5 cm. The baby’s gender, while not yet fully formed, are developed enough to reveal whether the fetus is a boy or a girl. Usually between 16 and 18 weeks, he or she may let you in on the secret when an ultrasound is performed. 
 
 
 
 
The legs and arms are getting longer and the fetus is very active with lots of movements on ultrasound scan. From 20 weeks it may be possible to feel movements which start as light flutters and get stronger. The anatomy scan happens around this time and is a screening test to check that all is developing well. 
The next few weeks the fetus gains weight, there is increase in the amnioitc fluid volume and the placenta continues to grow. 
At 24 weeks, the fetus can hear sounds so it is a good time to start talking to the baby. 
At 27 weeks, the fetus weighs 1055 grams or 2.3 pounds. 
 
 
 
 
 
 
See our FAQ's about our scans and also directions. 
 

The Third Trimester   

Before Birth  
 
The last three months is a time of growth 
The fetus is fully formed 
The fetus continues to move and strengthen its muscles. 
The bones are starting to harden but the skull remains soft. 
At 29 weeks eyes can open and close and sense changes in light. 
 
 
By 32 weeks hair has started to grow. 
The lungs are continuing to develop and the fetus will practice "breathing" movements . 
The average fetus will put on around half pound a week during the last 4 weeks. 
From 36 weeks the fetus typically adopts a head down position (cephalic presentation) 
The fetus is full term at 40 weeks. term. 
By 40 weeks the average fetus is 52 cm long and weighs 3619 grams or 8 pounds but of course fetus sizes will vary the midwife or obstetrician will have been assessing this during the pregnancy.  
Women may start to feel pressure symptoms as the fetus starts to move down and becomes engaged. 
Women may also start to experience Brackston Hicks tightenings as the womb prepares for birth.  
Antenatal classes at this time are really useful for mums to be to prepare for birth.  
 
See our FAQ's about our scans and also directions. 
 
This is for information only and does not consititute medical advice 
 
References 
http://perinatology.com/Reference/Fetal%20development.htm 
Babycenter. Fetal development week by week  
WomensHealth.gov. Stages of pregnancy  
 
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