The endlessly fascinating cycle of life begins when one from among millions of sperms fertilizes the female egg and the zygote is formed. The development of fetus, which begins as a zygote, develops into a morula or pre-embryo, then becomes a blastocyst, which goes on to become an embryo and finally a fetus goes something like this:
Development of fetus in the first trimester: Near the end of the first trimester, at 10 weeks the embryo developed into what is known as a fetus.
At 10 weeks gestation, the tiny fetus has a primitive brain and appears more like a primate than human and most of it is still head. The sex of the fetus can be determined by way of ultrasound at 10 weeks.
At the end of the first trimester, the fetus is about three inches long and it would typically weigh about one ounce.
The bones begin to develop about now. Involuntary movements and twitches can occur as the mental pathways start to develop.
Development of fetus in the second trimester: The facial characteristics of the nose, eyes, mouth etc now starts to develop. The first hair and lanugo (that disappears at birth) will start to develop during the first part of the second trimester. By about 15 weeks, it is thought, the fetus can taste what the mother eats.
The circulatory and digestive systems of the baby develop by about the middle of the second trimester. By about 18 weeks, the fetus is about 6 inches long and the sex organs have now developed.
When you are 20 to 22 weeks pregnant, the first fluttering movements of the baby can be perceived deep within by the mother. The baby develops fat layers and a layer of vernix.
The baby practices breathing by taking in and exhaling the amniotic fluid that surrounds it. It can hear you now and even perhaps recognize familiar voices and sounds.
Development of fetus in the third trimester: The tiny baby now weighs about 2 pounds at the start of the third and final trimester. It is about 27 inches in length and the eyes are just about starting to open.
The brain develops fast now and this is a good time to talk and sing to the baby. Exposure to music, reading to the baby is a good idea right now.
The baby puts on weight and during the second half of the third trimester it may be noticed that the movements were felt as fewer now as the baby has less space to move freely.
Most babies also get into the correct birthing position, head down, by the 33rd week or so. By the 36th or 37th week women tend to notice the ‘lightening’ which is the baby’s head settling and getting ready to come out of the birth canal.
By the 40th week a baby is usually about seven and a half pounds and is about 20 inches in length.