Fetal Development: Stages of Growth

Posted On Sep 24, 2021

Dr. Pundalik Baliga

Consultant - Fetal Medicine

Manipal Hospitals - Mangalore

Fetal Development and Growth

Every expecting couple in their journey of pregnancy, go through a myriad of emotions starting with the anxiety/stress in the periconceptional period until the conception is confirmed, followed by the happiness of having a healthy fetus inside her womb on serial scans, ending with the ultimate joy of the first glimpse and cry of the healthy newborn after delivery. Through this journey, the couple has many doubts and apprehensions regarding fetal development – its growth inside and normalcy. This article is an attempt to answer all these common queries and help the expecting couple to understand the development of their forthcoming bundle of joy better. 

Each month, a woman’s body goes through a reproductive cycle that can end in two ways - either a menstrual period or pregnancy. This cycle is continuously happening during the woman’s reproductive years - from puberty in her teens to menopause around the age of 50.

For a cycle to end in pregnancy, several processes/steps have to take place. First, a group of eggs (called oocytes) get ready to leave the ovary for ovulation (release of the egg). The eggs develop in follicles, which are small, fluid-filled cysts. Imagine these follicles as small containers for each immature egg. One egg will mature and continue the cycle from this group of eggs. This follicle then suppresses the rest of the follicles in the group and the other follicles stop growing at this point.

The mature follicle now opens and the egg is released from the ovary. This is called ovulation. Ovulation generally happens about two weeks before the next menstrual period begins. It generally corresponds to the middle of the menstrual cycle. This egg then combines with sperm, a process called fertilization, which develops into an embryo and further grows to become a baby over a period of 9 months. When there are no sperms to fertilize the egg, it degenerates and the woman experiences her menstrual period.

After ovulation, the opened (ruptured) follicle develops into a structure called the corpus luteum, which secretes (releases) the hormones progesterone and estrogen. Progesterone helps prepare the endometrium (lining of the uterus) for an expected pregnancy. This lining is the place where a fertilized egg settles to develop. If the woman does not become pregnant during a cycle, this lining is what is shed during the menstrual period.

Fertilization usually takes place about two weeks after the last menstrual period around the time of ovulation. When the sperm enters the egg, some changes develop in the protein coating of the egg to prevent other sperm from entering. Thus only one egg and one sperm combine together to form an embryo.

The actual embryo or fetal age (also known as conceptual age) is the amount of time that has passed since the egg was fertilized near the time of ovulation. However, because most women are not aware when the ovulation has occurred, but do know when their last period began, the time elapsed since the first day of the last normal menstrual period, known as the menstrual age, is used to determine the age of a pregnancy. The menstrual age is also termed as gestational age. It’s approximately about two weeks ahead of when conception actually occurred. The healthcare provider/ obstetrician will ask about this date and will use it to figure out how far the pregnancy has progressed. Traditionally, gestational age is expressed as completed weeks. Therefore, a 36 weeks + 2 days fetus is considered to be a 36-week fetus.

At the moment of fertilization, the baby’s genetic make-up is complete, including its sex. The gender of the baby depends on what sperm fertilizes the egg at the moment of conception. Women have the XX genetic combination, while men have the XY genetic combination. Hence the mother always generates an egg with “X”. The sperm can be a "X" or a "Y." If the sperm with an “X” fertilizes the egg, it forms XX which grows into a girl and if a sperm with a “Y” fertilizes the egg, it forms XY which grows into a boy.

Within 24 hours after fertilization, the fertilized egg that would be developing into a baby rapidly divides into many cells, called embryos. By the eighth week of pregnancy, the embryo grows and starts looking like a baby and is called a fetus. There are about 40 weeks in a typical pregnancy, which are divided into three trimesters as described below. 

Trimester Week
First Trimester
1 2 3 4 5 6 7 8 9 10 11 12 13
Second Trimester 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Third Trimester 28 29 30 31 32 33 34 35 36 37 38 39 40 41


During these 40 weeks, the fetus keeps growing and developing all its organs and structures and finally becomes the baby, which the expecting couple is so eager to welcome into this world.

Here are a few FAQs, addressing the common queries of the expecting couple.

How early can I know I’m pregnant?

From the moment of conception, the hormone human chorionic gonadotropin (hCG) will be present in the mother’s blood. This hormone is secreted by the cells that form the placenta (food source for your baby in the womb). It's also the hormone found in pregnancy tests. Even though this hormone is there from the beginning, it takes time for it to increase to detectable values within the mother’s body. It usually takes 3 to 4 weeks from the first day of your last period for the hCG level to rise sufficiently to be detected by pregnancy tests. hCG is detected in maternal urine between 6 and 14 days after fertilization (3 to 4 weeks gestational age). Fatigue and growing/enlarged or tender breasts may sometimes be the first signs of pregnancy.

When should I consult with my healthcare provider about a new pregnancy?

Most healthcare providers/ obstetricians will advise you to come in for an appointment once there is a positive home pregnancy test. These tests are very accurate once the woman has enough hCG circulating in her body. The doctors generally prescribe a vitamin supplement called folic acid during this stage, which has to be taken periconceptionally (when planning a pregnancy) also. It’s important that the mother gets at least 400 micrograms of folic acid each day during the pregnancy to make sure the baby’s neural tube (beginning/origin of the baby’s brain and spine) develops correctly. Many doctors would suggest that you take folic acid 2-3 months before planning a pregnancy. 

What’s the timeline for my baby’s development?

During a typical pregnancy, the baby will go through a lot of changes. The doctor will generally talk about the baby’s development in terms of weeks. So, if the expecting mother is three months pregnant, then she is about 12 weeks. Traditionally, we think of pregnancy as a nine-month process. However, this is not always the case. A full-term pregnancy is 40 weeks or 280 days. There will be distinct changes in the fetus and the mother, during each trimester.

Visit the Best Hospital for Fetal Medicine in Mangalore, India if you are facing any difficulties in conceiving.

Stages of Growth Month-by-Month in Pregnancy

First Trimester:

  • The first trimester will last from conception to 12 weeks. This is usually the first three months of pregnancy. During this trimester, the baby grows from a small cluster of cells to a fetus that is starting to have a distinct human appearance. Many birth defects occur in the developing embryo during this critical period. Most of these birth defects will have no known cause or may occur due to a combination of factors (multifactorial).

Month 1 (weeks 1 through 4)

As the fertilized egg develops and grows, a water-tight sac forms around it and gradually fills with fluid. This is known as the amniotic sac, and it serves as a cushion for the developing embryo.

During this time, the placenta also develops. The placenta is a round, flat organ that transports nutrients from the mother to the baby while also removing waste from the baby. Think of the placenta as a food source for the baby throughout the pregnancy.

In these first few weeks, a primitive face will take shape. The mouth, lower jaw, and throat will start developing. Blood cells start taking shape, and circulation begins. The tiny "heart" tube will beat around 60-70 times per minute by the end of the fourth week.

By the end of 1st month, the baby is around ¼ inch long (smaller than rice grain).

Month 2 (weeks 5 through 8)

The Baby's facial features continue to develop. Each ear begins as a small skin fold on the side of the head. Tiny limb buds that eventually grow into arms and legs are formed. Fingers, toes, and eyes also start forming.

The neural tube (spinal cord, brain, and another central nervous system neural tissue) is now completely formed. The digestive tract and sensory organs begin to develop as well. Bone starts to replace cartilage.

The baby’s head is large in proportion to the rest of its body at this point. At about 6 weeks, a baby's heartbeat can usually be detected by an ultrasound scan. The fetal heart rate varies between 110 to 180 beats per minute throughout the pregnancy.

After the eighth week, the baby is called a fetus rather than an embryo.

By the end of 2nd month, the baby is around 1-inch long and weighs about 1/30 of an ounce (1 gram).

Crown-rump length (length of a baby from its head to buttocks) of 7 mm or greater with no heartbeat, or mean gestational sac diameter of 25 mm or greater with no embryo in it is considered consistent with early pregnancy loss/ failed pregnancy.

Month 3 (weeks 9 through 12)

The Baby's arms, hands, fingers, feet, and toes are fully formed during this period. At this stage, the baby is starting to explore a bit by doing things like opening and closing its fists and mouth. Toenails and fingernails start to develop and the external ears are formed. The teeth also start forming inside the gums which later erupt after birth.  The Baby's reproductive organs also develop during this period.

The baby is fully formed by the end of the third month. All the organs and limbs (extremities) are present and will develop further to become functional. The baby’s circulatory and urinary systems are also working and the liver starts producing bile.

In 12 weeks, the average fetus is 2.1 inches (5.3 cm) long and weighs around 2 ounces (50-60 grams).

Since the baby's most critical development has already occurred, the risk of miscarriage drops significantly after three months when the baby is healthy.

Second Trimester

  • The middle stage of pregnancy is frequently regarded as the most enjoyable part of the experience. By this time, any morning sickness that the mother might have experienced in the first trimester is probably gone and the discomfort of early pregnancy has faded. During this month, the baby's facial features will begin to develop. The mother may also start perceiving the baby’s movement as it flips and turns inside the uterus. A detailed anatomy scan (an ultrasound that checks the baby’s physical development) is done around 18-22 weeks.

Month 4 (weeks 13 through 16)

The baby's heartbeat may now be audible through a device known as a Doppler. The fingers and toes are distinct. Eyelids, eyebrows, eyelashes, hair, and nails develop. Teeth under the gums and bones become denser. The baby can even suck its thumb, yawn, stretch and make faces.

The nervous system is starting to function. The development of reproductive organs and genitalia is fully completed. By the end of the fourth month, the baby is about 6 inches long and weighs about 100-120 grams.

Month 5 (weeks 17 through 20)

At this point, the mother may notice the baby moving around. The baby starts developing muscles and exercising them. This first movement, known as quickening, can feel like a flutter.

Hair begins to grow on the baby's head. The baby's shoulders, back, and temples are covered by soft fine hair called lanugo. This hair protects the baby and is shed at the end of the baby's first week.

The baby's skin is covered in a whitish coating known as vernix caseosa. This "cheesy" substance is thought to protect the baby's skin from prolonged exposure to amniotic fluid.  This layer is shed shortly before birth.

The baby is about 10 inches long and weighs 300 to 450 grams by the end of the fifth month.

Detailed anatomical assessment of the fetus is typically done between 18 to 22 weeks by an ultrasound scan.

Month 6 (weeks 21 through 24)

If we could look inside the uterus at this stage, and see the baby with our naked eyes, we would see that the baby's skin is reddish in color, wrinkled with veins being visible through the baby's translucent skin. The Baby's finger and toe prints would also be visible.

During this stage, the eyelids begin to part and the baby can open its eyes. The baby reacts to sounds by moving or increasing its pulse rate. Jerky movements may also be noticed when the baby has hiccups.

By the end of the sixth month, the baby is about 12 inches long and weighs 500 to 700 grams.

Month 7 (weeks 25 through 28)

The baby will continue to mature and gain body fat reserves. The baby's hearing is fully developed at this point. The baby moves around frequently and responds to stimuli such as sound, pain, and light. The amniotic fluid begins to diminish.

 If born prematurely, the baby is more likely to survive after the seventh month (>26 weeks). 

At the end of the seventh month, the baby is about 14 inches long and weighs around 900 grams – 1.2 kg

Third Trimester

This is the final stage of the pregnancy. The expecting mother will undoubtedly be tempted to begin counting down the days until her due date in the hopes that it will arrive sooner, but each week of this final stage of development helps the baby prepare for childbirth. Throughout the third trimester, the baby will gain weight quickly, adding body fat that will help it sustain after birth.

Remember, even though popular culture mentions only nine months of pregnancy, the mother may actually be pregnant for 10 months. The typical, full-term pregnancy is 40 weeks, which can take the mother into the tenth month. It’s also possible that she can go past her due date by a week or two (41 or 42 weeks). The obstetrician will monitor the mother closely as she approaches her due date. If she passes her due date and doesn’t go into spontaneous labor, the health care provider may induce labor pains artificially with the help of medications. Make sure to talk to the obstetricians during this trimester about the birth plan. Also, the doctor might monitor the wellbeing of the baby with a non-stress test which monitors the variations in the baby’s heartbeat with each movement of the baby through a machine that gives a graphical representation of the variations in a fetal heartbeat.

Month 8 (weeks 29 through 32)

The baby will continue to mature and gain body fat reserves. The mother may notice her baby kicking more vigorously. The baby's brain is rapidly developing at this point, and it can see and hear. The majority of internal systems are mature, but the lungs may still be immature.

The baby is about 18 inches long and weighs as much as 4.5- 5 pounds (1.1 - 2.2 kg).

Month 9 (weeks 33 through 36)

In this stage, the baby will continue to grow and mature. At this point, the lungs are almost fully developed. The baby's reflexes are coordinated so it can blink, close its eyes, turn the head, grasp firmly, and respond to sounds, light, and touch. 

The baby is about 17 to 19 inches long and weighs from 5 to 6 pounds (2.2- 2.6 kg).

Month 10 (Weeks 37 through 40)

In this final month, the mother can go into labor at any time. She may notice that the baby moves slightly less due to tight/reduced space. At this point, the baby’s position may have changed to prepare for birth. Ideally, the baby is head down inside the uterus. The mother may feel quite uncomfortable in this final stretch of time as the baby descends lower into her pelvis and prepares for birth. 

The baby is ready to meet the outside world at this point. Being about 18 to 20 inches long and weighing about 6-7 pounds (2.5-3.5 kg).

Thus, with labor, the pregnancy and the baby’s sojourn inside the uterus ends and it is born into the world bringing with it lots of happiness to the expecting couple and their families.

Consult the Fetal Medicine Expert in Mangalore to discuss your options of getting pregnant.


Dr. Pundalik Baliga

Consultant - Fetal Medicine

Manipal Hospital, Mangalore


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