The unborn baby spends around 38 weeks in the womb, but the average length of pregnancy (gestation) is usually counted as 40 weeks. This is because pregnancy is counted from the first day of the woman's last period, not the date of conception, which generally occurs two weeks later. Pregnancy is divided into three trimesters:
* First trimester - conception to 12 weeks
* Second trimester - 12 to 24 weeks
* Third trimester - 24 to 40 weeks.
Thirty hours after conception, the cell splits into two. By day three, the cell (zygote) has divided into 16 cells. After two more days, the zygote has migrated from the fallopian tube to the uterus (womb). At day seven, the zygote burrows itself into the plump uterine lining (endometrium). The zygote is now known as a blastocyst.
The developing baby is tinier than a grain of rice. The rapidly dividing cells are in the process of forming the various body systems, including the digestive system.
The evolving neural tube will eventually become the central nervous system (brain and spinal cord).
The baby is now known as an embryo. It is around 3mm in length. By this stage, it is secreting special hormones that prevent the mother from having a menstrual period.
The heart is beating. The embryo has developed its placenta and amniotic sac. The placenta is burrowing into the uterine wall to access oxygen and nutrients from the mother's bloodstream.
The embryo is now around 1.3cm in length. The rapidly growing spinal cord looks like a tail. The head is disproportionately large.
The eyes, mouth and tongue are forming. The tiny muscles allow the embryo to start moving about. Blood cells are being made by the embryo's liver.
The embryo is now known as a foetus and is about 2.5cm in length. All of the bodily organs are formed. The hands and feet, which previously looked like nubs or paddles, are now evolving fingers and toes. The brain is active, and has brain waves.
Teeth are budding inside the gums. The tiny heart is developing further.
The fingers and toes are recognisable, but still stuck together with webs of skin.
The foetus can swim about quite vigorously. It is now more than 7cm in length.
The eyelids are fused over the fully developed eyes. The baby can now mutely cry, since it has vocal cords. It may even start sucking its thumb. The fingers and toes are growing nails.
The muscles develop further, and the baby's movements as it swims and kicks are more coordinated.
The foetus is around 14cm in length. Eyelashes and eyebrows have appeared, and the tongue has tastebuds. An ultrasound is commonly performed around this time (usually week 18) to check for abnormalities, position of placenta and multiple pregnancies. Interestingly, hiccoughs in the foetus can often be observed.
The foetus is around 21cm in length. The ears are fully functioning and can hear muffled sounds from the outside world. The fingertips have prints. The genitals can now be distinguished with an ultrasound scan.
The foetus is around 33cm in length. The fused eyelids now separate into upper and lower lids, enabling the baby to open and shut its eyes. The skin is covered in fine hair (lanugo) and protected by a layer of waxy secretion (vernix). The baby 'breathes' amniotic fluid in and out of its lungs.
The foetus is around 37cm in length. The growing body has caught up with the large head, and the baby now seems more in proportion.
The baby spends most of its time asleep. Its movements are strong and coordinated. It has probably assumed the 'head down' position by now, in preparation for birth.
The baby is around 46cm in length. It has probably nestled its head into its mother's pelvis, ready for birth. If it is born now, its chances for survival are excellent. Development of the lungs is rapid over the next few weeks.
The baby is around 51cm in length and ready to be born. It is thought that the baby secretes hormones that trigger the onset of labour.
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