Fetal Development Stages: Your Guide To The Complete Pregnancy Period

Pregnancy is a beautiful yet complicated period in a woman’s life. Your body goes through a metamorphosis of sorts as you birth a new life into this world.

There are also bodily changes – from morning sickness in the first trimester, the belly bump in the second, to of course the contractions and labour pain in the third. The process is anything but easy.

On a cellular level though, the model is continuously going through changes that lead to your baby coming to life. And this process begins right after fertilisation when the sperm attaches to the egg.

The 40-week period of a pregnancy is full of exciting developments where the sperm transforms into a foetus followed by an embryo and then into a living and breathing baby cocooned in the womb till the labour day.

Pregnancy can be further divided into three major periods or three trimesters, while the foetus itself goes through a number of changes through this period.

These are the fetal development stages and here’s a quick explainer on how the process works. For new mums out there expecting their first baby, this guide will help you understand the pregnancy period more clearly.

When Does A Pregnancy Start?

The first day of your last menstrual period marks the first day of your pregnancy. This is the gestational age or menstrual age and the conception will typically occur two weeks ahead.

Now, this date is particularly important as that will help your gynaecologist determine your due date and how far along you’ve come in your pregnancy.

What Happens During Conception?

During your menstrual cycle every month, the period can either end with you having another menstrual period or becoming pregnant.

When you try to have a baby during your most fertile period (there are ways you can get pregnant even during the low fertility period), a group of eggs termed oocytes get ready to leave the ovary for ovulation. This is when the egg releases and develop in small fluid-filled cysts termed follicles.

These are essentially containers for each immature egg. One egg out of this group will mature and continue through the cycle. The mature follicle will then release the egg from the ovary.

This is the ovulation process and it typically takes place two weeks before your next menstrual period.

Once the ovulation period is complete, your body secretes the progesterone hormone that creates a lining on the uterus where the egg settles in the case of a pregnancy. When you have sex during the fertile window, the sperm has a chance to penetrate the egg.

The Process After Conception

The pregnancy process begins 24 hours after fertilisation when the eggs start to divide themselves into many cells. It will remain in the fallopian tube for about three days after conception.

The egg then passes through the fallopian tube to the uterus and then attaches to the endometrium. This is implantation.

Prior to implantation, the blastocyte breaks out of its protective cover and makes contact with the endometrium. At this point, the endometrium becomes thicker, while the body seals the cervix by a plug of mucus.

In the following three-week period, the blastocyte cells turn into a little ball that’s known as the embryo. The embryo’s cells have formed by this period and it is rapidly going changes of its own during the first few weeks of pregnancy.

What Are The Fetal Development Stages

Once you’ve entered the fetal stage of the pregnancy, it will keep changing throughout the 40-week period. This period is further divided into three trimesters, and each trimester brings its own distinct changes.

Mums should also know that the 40-week period, although ideal, isn’t set in stone. Some mums undergo a preterm delivery where the baby is born before the 40-week mark. In some cases, you could also have a post-term delivery where the baby is born after the 40th week.

Here’s how the final weeks of the pregnancy period, more commonly known as the gestational period, look like:

The last few weeks of pregnancy are divided into the following groups:

• Early term: 37 0/7 weeks through 38 6/7 weeks.
• Full term: 39 0/7 weeks through 40 6/7 weeks.
• Late-term: 41 0/7 weeks through 41 6/7 weeks.
• Post-term: 42 0/7 weeks and on.

Fetal Development Stages Of Growth

First trimester

These are the first 12 weeks of pregnancy as we begin the fetal development stages. During this period, the fertilised egg changes from a small group of cells into a foetus.

Month 1 (Week 1-4)

1. In the first month, the fertilised egg grows and a water-tight sac forms around the embryo. This is the amniotic sac filled with amniotic fluid and it helps protect the baby.
2. This is also the period when the placenta develops. It’s round, flat organ that transfers nutrients from the mother to the baby. It also transfers wastes from the foetus.
3. The embryo will begin taking shape during this period into the human form that we first see in sonography scans. This includes large dark circles for the eyes, a mouth, lower jaw and throat. The blood cells take shape and the circulation begins as well.
4. By the end of the fourth week, the baby’s tiny heart will beat 65 times a minute. And all of this happens in a size as small grain or rise of 1/4 inch long.

Month 2 (Week 5–8)

1. You will now begin to see more prominent features being developed. This includes the ears that start off as a little fold of skin at the side of the head.
2. You will also notice tiny buds that form into arms and legs eventually. They further make way for fingers, toes, while the eyes are now taking shape.
3. By the second month, the neural tube comprising the brain, spinal cord and the entire central nervous system has been formed. The digestive tract and sensory organs begin to develop now, while the bone starts to replace the cartilage.
4. This is also the period when the head is larger than the rest of the body in terms of size. At about the six week mark, the doctor will be able to detect a heartbeat.
5. At the eighth week mark, the embryo turns into a foetus.
6. By the end of the second month, the baby has grown about one inch long and 1/30 of an ounce in weight. Yes, that’s very tiny and precious too.

Month 3 (Week 9-12)

1. At this stage, the arms, hands, fingers, feet have been fully developed and the foetus is now doing things like opening its mouth or closing the fists.
2. The fingernails and toenails start developing as well, while the ears have been formed as well.
3. This is also when the teeth start forming under the gums, which we don’t see until about nine months after birth.
4. The foetus’ reproductive organs also begin developing during this stage. However, determining the gender of the baby is still some time away.
5. At the end of the third month, the foetus has been fully developed with organs, limbs, circulatory and urinary systems, while the liver produces bile.
6. The baby at this stage is about four inches long and weighs about one ounce.

Do note, this also completes the first trimester period. It’s also the time when expecting mums need to take extra care as the risk of miscarriage is considerably high during this period.

Second Trimester

Moving to months four to seven. This period is often described as the best part of the pregnancy. The morning sickness, nausea, vomiting usually go away by now.

You will also start noticing the foetus’ movement in your belly and can also determine the gender of the little one around the 20-week mark.

Month 4 (Week 13-16)

1. During the fourth month of the pregnancy, the baby’s heartbeat will be audible using the doppler.
2. The fingers and toes are now well-defined while the formation of eyelids, eyebrows, eyelashes, nails and hair is complete as well.
3. The foetus begins to suck its thumb, stretch, yawn, and even make faces. Their teeth and bones will also become denser during the period.
4. The foetus’ nervous system starts functioning and the reproductive organs and genitals have been fully developed.
5. At the end of the fourth month, the foetus is about six inches in length and four ounces in weight.

Month 5 (Week 17-20)

1. The fifth month will have the foetus moving around. It’s developing muscles and exercises them, which causes the movement.
2. This is also the period where hair begins to grow on the head, while the shoulders, back and temples get covered in lanugo, a soft fine hair. This hair protects the foetus during the pregnancy period and will shed on its own in the first weeks after birth.
3. The skin also gets covered in vernix caesosa, a whitish coating that protects the fetal skin from long exposure to the amniotic fluid. This coating sheds just before birth.
4. At the end of the fifth month, the foetus is about 10 inches long and weighs about half to one pound.

Month 6 (Week 21-24)

1. The foetus’ skin starts turning reddish in colour at this stage. You can also see the veins visible through the translucent skin.
2. The finger and toe prints are visible as well, and the eyelids begin to part and the eyes open.
3. The fetus responds to sounds by moving or increasing its pulse.
4. At the end of the sixth month, the foetus measures 12 inches in length and weighs about two pounds.

Month 7 (Week 25-28)

1. The foetus continues to mature and begins developing reserves of body fat. The hearing develops fully by the seventh month, while it also changes positions frequently.
2. It will also respond to stimuli like sound, light and pain.
3. The amniotic fluid begins to reduce in the sac.
4. At the end of the seventh month, the fetus is about 14 inches long and weighs about 2-4 pounds.

The seventh month is particularly crucial since there is a risk of premature delivery but the baby will survive the early birth.

Third Trimester

Now comes the final weeks of the fetal development stages as the countdown begins for labour day. The final fetal development stage prepares the foetus for birth.

The third-trimester witnesses a sharp weight gain in the foetus, adding body fat that will help after birth.

Do note that a full-term comprises a 40 week period but your pregnancy could stretch by a week or two, or into the tenth month. Do remember, this is completely normal.

Your gynaecologist will continuously monitor you during this period and will recommend in case you need to have induced labour. There could also be the possibility of a C-section.

Month 8 (Week 32)

1. There is a period when the foetus continues to mature and develops body fat reserves.
2. The foetus will kick and move more often, while brain development takes place rapidly.
3. The foetus can see and hear, while most of the internal systems have been well developed. The lungs though may still be a while from being fully functional.
4. At the end of the eighth month, the foetus grows up to 18 inches in length and weighs about five pounds.

Month 9 (Week 33-36)

1. The lungs come close to being fully developed by this time.
2. The foetus can make coordinated reflexes like blink, close its eyes, turn the head and respond to sounds.
3. At the end of the ninth month, the foetus is about 17-19 inches in length and will weigh about 5.5 pounds to 6.5 pounds.

Month 10 (Weeks 37-40)

1. It’s now time to await the due date that can kick in at any time. The baby is ready come to out into the world and the foetus’ position may have changed to prepare for delivery.
2. Ideally, it’s head down in the uterus.
3. At the end of the 40th week, the foetus is about 18 to 20 inches in length, while the ideal weight is about 7 pounds.

Source: Myclevelandclinic.org

ALSO READ:

Premature Birth May Be Reduced By Treating Foetus As Patient: Study

You, Him, And The Foetus: Myths And Facts About Pregnancy Sex