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Fetal Development & Human Gestational Age- Explained by Gynaecologist at the CK Birla Hospital

fetal development process

When a newly pregnant woman gets her first sonogram the doctor shows her a small little blip growing inside her womb. This is by far the most emotional moment for her, as it is the first time she sees her baby. In the first trimester, the baby is just a foetus, ready to grow into an infant and start a new life. This entire process of a foetus turning into a baby is called foetal development. This is a critical part of the entire pregnancy journey and needs proper monitoring by an experienced maternity team comprising the doctor and the ultrasound technicians who will be able to track the growth of the foetus inside the womb. 

As an informed parent-to-be, it helps to know how your baby is supposed to develop. This is the time when all the organs and the body of the baby develop with which it will arrive into this world. Different growth trends can be noted in some babies so the timelines depend on each baby. Having said that, to know how a foetus usually grows, read on…

What is Foetal Development?

At an average of two weeks from your last menstrual period, your egg will fertilise, and you will conceive. Within the first 24 hours of fertilisation, cell division begins on the fertilised egg. Then it moves down the fallopian tube as a blastocyst, develops into an embryo and simultaneously the link between your cervix and birth canal is sealed by a layer of mucus. By the 8th week of pregnancy, the embryo evolves into a foetus. At this time the hormone human chorionic gonadotrophin (HCG) will start showing up in your blood works confirming your pregnancy.
An average full pregnancy takes about 40 weeks which is divided into 3 trimesters of roughly 12-13 weeks/ 3 months each.

What’s the Timeline for Foetal Development?

You and your foetus undergo a lot of changes throughout each trimester and your maternity team will be able to monitor the growth on a weekly basis. Depending on the calendar months you are pregnant, the entire pregnancy can last from 9 to 10 months which is normal.

The closer you get to the end of your pregnancy, the more critical it becomes to track the changes in the baby’s growth. The last few weeks of pregnancy are vital because babies born before full term have a higher risk of breathing, hearing or learning issues than babies born in the full term.

The last few weeks of pregnancy are divided into 4 groups, each showing the first number as the number of weeks a.k.a. Gestational age, followed by days in a week. So 6/7 means the 6th day of the week.

Term of pregnancy Time Period of Human Gestational Age
Early term 37 0/7 weeks to 38 6/7 weeks.
Full term 39 0/7 weeks to 40 6/7 weeks.
Late-term 41 0/7 weeks to 41 6/7 weeks.
Post-term 42 0/7 weeks and on

What Happens in Each Stage of Foetal Development?

First Trimester:

This will last for 3 months or 12 weeks. The fertilised egg will evolve from a cell to a blastocyst, then an embryo and finally a foetus that starts developing the baby’s features.

  1. Month 1 (Weeks 1 to 4): Growth: up to ¼ inches
    The fertilised egg grows, and a watertight barrier is formed around it which starts filling up with fluid called an amniotic sac which cushions the embryo. Then the placenta develops which takes nutrients from the mother’s body and transfers wastes from the foetus throughout the pregnancy. The baby will now have a primitive face. The blood cells are developing and a tiny heart will beat 65 times per minute.
  2. Month 2 (Weeks 5 to 8): Growth: up to 1 inch long
    By now the face develops and the ears start showing. The legs, arms, fingers, toes and eyes are also forming. The nervous system is now a neural tube containing the brain, spinal cord and tissues. The bones replace cartilage, while the digestive tract and sensory organs develop. The head is bigger than the rest of the body and heartbeats can be detected.
  3. Month 3 (weeks 9 to 12): Growth: up to 4 inches long
    The foetus and its arms, hands, fingers, feet and toes, which were tiny nubs, are now fully formed. The foetus can now open and close its fists and mouth. Nails are developing and the ears are formed. Teeth start forming under the gums. The reproductive organs are yet to be properly distinguished on ultrasound. By the 12th week, all limbs are visible and the circulatory as well as urinary systems are functional, while the liver starts producing bile.

Second Trimester:

At this stage, the risk of miscarriage has dropped by half, morning sickness is gone and you feel much more comfortable now. The baby’s facial features are starting to develop and the foetus is able to flip and turn.

  1. Month 4 (weeks 13 to 16): Growth: up to 6 inches long
    The heartbeats are audible through a Doppler. The fingers and toes are defined. Eyes including brows and lashes show up and nails and hair are properly formed. The foetus can yawn, stretch and even make faces. The nervous system is fully functional and the reproductive system is up and running.
  2. Month 5 (weeks 17 to 20): Growth: up to 10 inches long
    The foetus now has muscles and can move around. This first movement is called quickening. Hair grows on the head and shoulders, back as well as temples. This soft fine hair is called lanugo and it protects the foetus till the first week of birth. The skin grows a protective coating called vernix caseosa, which protects the skin from amniotic fluid exposure, till the baby is born.
  3. Month 6 (weeks 21 to 24): Growth: 12 inches long
    The foetus now has reddish skin, and wrinkles and veins are visible through the skin. The foetus is now able to open its eyes, respond to sounds and sometimes jerk when it hiccups. At the end of the period, if there is a premature delivery, the baby can survive with critical care at the NICU.
  4. Month 7 (weeks 25 to 28): Growth: 14 inches long
    The foetus continues to mature and develop reserves of body fat. At this point, the hearing is fully developed. The foetus changes position frequently and responds to stimuli, including sound, pain and light. The amniotic fluid begins to diminish.

Third Trimester:

You are in the endgame now! The final leg of your pregnancy can make you anxious as the foetus gains weight quickly with body fat that will help the baby grow after birth. The duration of the last trimester can last up to 10 months, which is completely normal. In case you pass your due date without spontaneous labour, your doctor might induce your labour through medication. Make sure your maternity team is well prepared for the main event.

  1. Month 8 (weeks 29 to 32): Growth: up to 18 inches long
    The foetus starts to develop reserves of body fat. The other systems except the lungs have fully matured. There might be more kicking and the foetus will be able to see and hear properly as the brain is now developing rapidly. The weight of the baby is around 5 pounds.
  2. Month 9 (weeks 33 to 36): Growth: 17 to 19 inches long
    The body keeps growing to its full potential and the lungs are almost developed. Its reflexes like blinking, grasping, turning, and response to light and touch are developing fast. Your belly has now reached its full potential too as the baby now weighs around 5 ½ to 6 ½ pounds.
  3. Month 10 (Weeks 37 to 40): Growth: Up to 20 inches long
    The space is tight, the baby can come out anytime, so the foetus must position itself for birth. Ideally, the head should be the first to come out. The pelvis prepares for birthing and based on the condition of the mother a normal or caesarean section is performed.

To summarise, this is how your baby develops in your womb and the whole experience of feeling the kicks and the hiccups can be extremely emotional for the parents-to-be. At this precious time in your lives, it is essential that you have a maternity team ready to be by your side at every beck and call to ensure a smooth and complication-free delivery. If proper monitoring is done then the baby and the mother can be saved from multiple fatalities at birth. Our team at the Mother & Child Department of the CK Birla Hospital is prepared to handle any sudden requirements that may crop up to assist you in your pregnancy journey. Book an appointment Now.

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