Fetal Development: How a Fetus Develops in Mother’s Womb?

Editor's Note: This article has been recently updated with latest information and research studies.
 

When pregnancy becomes official, not only mothers experience rapid bodily changes, suffer the notorious symptoms and endure the emotional rollercoaster ride associated with pregnancy, but much more to the developing baby. The baby undergoes a drastic evolution that is, from reptilian look to humanoid. This article is a rough guide to moms-to-be about how a baby is growing in a mother’s womb week by week.

The Earliest Days of Fetal Development

Before delving into the monthly and weekly fetal development, here’s a primer on conception:

When the man ejaculates, the semen — which contains millions of sperm cells — is deposited into the vagina. They travel through the cervix and stand by for the woman’s egg cell. When they meet, fertilization happens. The fertilized ovum is called a zygote – the earliest form of a baby. The zygote contains DNA or genetic information that comes from the parents – half of it from the mother and the rest from the father. At this period, the zygote turns into a blastocyte which go through a series of cell division resulting in a group of cells with an inner shell (embryo) and outer shell (fetal membrane). The blastocyte will then be implanted into the uterine wall approximately 6 days after conception. After differentiation occurs, the blastocyte is now referred to as embryo.

Fetal Development Month by Month Changes

Fetal Period Approximate Length Significant Development
First Month(4 weeks to 7 Weeks) 0.6 cm The heart begins to beat.
Second Month(8 weeks to 12 weeks) 2.5 cm The embryo is officially called fetus.
Third Month(13 weeks to 16 weeks) 7.5 cm The sex of the baby is determinable.
Fourth Month(17 weeks to 20 weeks) 15 cm The fetus starts to create subtle movements such as kicking.
Fifth Month(21 weeks to 24 weeks) 25 cm Fine, black hair called lanugo develops. The fetus also now assumes the thumb-sucking position, and heartbeats can now be heard via ultrasound.
Sixth Month(25 weeks to 29 weeks) 30 cm The eyes of the baby start to open.
Seventh Month(30 weeks to 33 Weeks) 40 cm The fetus is fully developed.
Eight Month(34 weeks to 38 weeks) 46 cm The fetus elicits strong and regular fetal activity.
Ninth Month(39 weeks to 40 weeks) 50 cm The fetus lungs are mature. The baby may come out anytime.

Fetal Development Weekly Changes

Fetal development is divided into two periods — the embryonic period which takes place between 5 and 10 weeks, and the fetal period which occurs between 11 and 42 weeks. The fetal period is when the placenta replaces the yolk sack in providing nourishment to the baby.

Fact: Placenta begins to grow during implantation and becomes fully developed at approximately 12 weeks.

FETAL DEVELOPMENT
Period (Week) Major Changes
5th week The central nervous system (brain and spinal cord), the heart and GI tract begin to form.
6th week The baby has now a pre-formed face.
7th week Majority of the development takes place on the baby’s head. The eyes and ears are becoming noticeable and the mouth starts molding. The liver is also working out to produce red blood cells until the bone marrow fully matures.
8th week Baby’s brain activity starts to function. The nose is coming out visible so as well with the limbs. Moreover, the baby at this point has developed retinas and lens. The heart forms into two chambers.
9th week The limbs are becoming longer. The baby already has fingers but no touch pads. The neck is beginning to outline and is visible via ultrasound. Bone ossification may occur.
10th week The fingers and toes are fully developed (webbing is entirely gone). The hair is growing and the baby gets its own eye color. The baby is also taking up fluid.
11th week The baby exhibits swallow reflex. Some bones also start to harden. The baby is now performing somersaults inside the womb.
12th week The baby starts opening and closing his/her eyes. The brain cells continue to multiply, and the synapses are forming swiftly.
13th week The baby has already tiny fingertips, and the fats are entirely gone making the bones more prominent. In addition, the baby swallows the amniotic fluid.
14th week The baby reacts to external stimulus and can exhibit facial expressions such as frowning.
15th week The baby can sense the light and reacts to it by moving away from the source.
16th week Patterning of scalp is observed.
17th week Baby’s external ears are moved into their right and final positions.
18th week Myelin production commences. Myelin is a covering that protects cells in the nervous system.
19th week Vernix caseosa, or the cheese-white, waxy substance that covers the baby, forms.
20th week Meconium or dark-stained stools is produced.
21th week The baby starts to blink.
22nd week Lanugo covers the entire body. Eye lashes appear. The baby is active and fetal movements may now be felt by the mother.
23rd week Because the baby can now hear clearly, this is the best time to read stories or play music for him/her.
24th week The baby’s respiratory system now develops into branches.
25th week The baby learns to respond to touch.
26th week All the eye parts of the baby are fully formed. Air sacs in lungs continue to develop.
27th week Baby’s lungs still immature but start functioning.
28th week Bones are nearly developed.
29th week Clitoris is prominent (baby girl). The testes are starting to descent (baby boy).
30th week The baby can distinguish light from dark.
31st week The baby looks nearly like a newborn.
32nd week Baby skin is becoming smooth and soft.
33rd week The head starts moving down to prepare for delivery.
34th week Hearing is fully developed.
35th week Fingernails and toe nails are in full length.
36th week The lanugo and vernix caseosa are nearly all shed.
37th week The baby has full head of hair
38th week Laungo completely disappears while the body fat increases*.
39th week Baby’s organs have all matured.
40th week The baby is readily prepared for birth.

Tracking fetal growth and development is an exciting activity. Learn about the physiologic changes and activities of the baby while he/she is in the womb and get expert advice on how to endure safe pregnancy.

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Author

Expert Author : Peony C Echavez (Consumer Health Digest)

Peony is a registered nurse, and former Director of Nursing services for a large nursing facility. She has written web content for a large health education website, and currently creates content for a number of health practisioners.