Pregnancy Stages: Changes In The Mother’s Body And Fetal Development

For a pregnant woman, giving life to a new creature inside your body is an amazing life experience, even though you may face difficulties at some point along the way.

Pregnancy not only differs from woman to woman but is also different for the same mother from one pregnancy to the other one. Some pregnancy symptoms may last for several weeks or months, whereas others may be shorter or might not affect you at all.

Even though the unborn baby spends around 37 weeks in the uterus (womb), the average length of human gestation is calculated to last about 280 days or 40 weeks. This is because the pregnancy length is counted from the first day of the woman’s last menstrual period, which happens two weeks before conception actually occurs.

Pregnancy is divided into three trimesters, with each lasting between 12 and 13 weeks. Each trimester is characterized by different changes in your body as well as in the developing embryo/fetus, and a summary of these changes will be discussed below.

Pregnancy stages
Credit: Pixabay


Pregnancy Stages


Conception and implantation

About two weeks after your last period, you ovulate and your ovaries release one mature egg. The egg can be fertilized for 12 to 24 hours after being released as it goes down the fallopian tube toward the uterus.

If an egg meets up with a sperm cell inside the fallopian tube, it will combine into one single cell, a process known as fertilization or conception.

At this process, depending on what chromosome (X or Y) the egg will receive from the sperm cell, the sex of the fetus will be determined. If the egg receives an X chromosome, the baby will be a girl; if the egg receives a Y chromosome, the baby will be a boy.

According to the Cleveland Clinic, it will take about three to four days for the embryo (the fertilized egg) to move to the lining of the uterus, where it implants to the uterine wall. After the fertilized egg is implanted, the cells start to grow and form the fetus and the placenta. The placenta is the organ that provides oxygen and nutrients to your growing baby and removes waste products from your baby’s blood.

Pregnancy stages
Credit: Nataliya Iakubovskaia / iStock / Getty Images Plus

First trimester (weeks 1-12)

Changes in the mother’s body

During the first trimester, you will experience many pregnancy symptoms as you adjust to the hormonal changes of pregnancy. In the early weeks, the most changes to occur are the ones inside your body.

One of the changes is the hCG hormone (human chorionic gonadotrophin ) which will be produced in your body from the time conception occurs. So, hCG levels are the hormone of interest during pregnancy, as its levels are what pregnancy tests measure to tell you whether you are pregnant or not. Levels of this hormone can be detected in your urine about a week after a missed period.

Other hormonal changes you will experience during pregnancy are waves of nausea and vomiting a.k.a the morning sickness, which is caused by the rising levels of estrogen and hCG. Despite its name, morning sickness can occur at any time of the day.

During pregnancy, you will also feel more tired than usual, due to the rising levels of the hormone progesterone, which increases sleepiness.

Other symptoms that follow are frequent urination, due to the enlargement of the uterus which puts pressure on the bladder.

Your breasts will feel more tender and swollen, the nipples will darken and enlarge.

Your digestive system may slow down to increase the absorption of beneficial nutrients, and your heartbeat will increase to pump more blood to the uterus, which will supply it to the fetus.

Besides the physical changes, you will also experience mood swings and forgetfulness in the early months of pregnancy and throughout it.

Pregnancy stages
Credit: Pixabay

First trimester

Development of embryo/fetus

An embryo is called a developing baby from the conception time until the eighth week of pregnancy.

According to the American College of Obstetricians and Gynecologists (ACOG), the heart, the lungs, the arms, legs, brain, spinal cord and nerves of the baby begin to develop during the first month of pregnancy.

Whereas during the second month of pregnancy, all major body organs and systems, such as the ears, ankles, wrists, fingers, and toes are formed but not completely developed. The eyelids also form and grow but are sealed shut.

The developing baby is called a “fetus” after 8 weeks as an embryo.

During this period, the fingers and toes of the fetus have soft nails, the mouth has 20 buds that will become baby teeth, and the skin is almost transparent.

You can hear your baby’s heartbeat for the first time sometime between the 10th and the 12th week.

At this stage of pregnancy, it’s crucial that you do not take any harmful medication, such as illegal drugs, as the first trimester is the period when most miscarriages and congenital disabilities occur.


Second trimester (weeks 13-27)

Changes in the mother’s body

By the second trimester, your body will adjust to changing hormone levels, so some of the symptoms of early pregnancy may begin to ease or disappear.

However, as the fetus is continually developing, other symptoms may appear. Some of them are the pelvic pressure, a more visible baby bump, and back pain.

According to Burch, sometime between the 16th and 18th weeks of pregnancy, you may feel the first fetus movements, known as quickening.


Development of the fetus

In the second trimester, the fetus will be between 3 and 5 inches long. Sometime between 18 and 22 weeks, if you want to know the sex of the baby in advance, you may know through an ultrasound.

According to ACOG, during the fourth month, the fetus can bend its arms and legs and can swallow and hear. Also, the kidneys start working and the neck, fingernails, eyebrows, and eyelashes form.

By the fifth month, the fetus sleeps and wakes on regular cycles, while a fine hair and a waxy coating are formed to cover and protect its thin skin.

During the sixth month, the hair begins to grow and eyes begin to open, while the brain is swiftly developing. As for the lungs, although they are formed during this period, they can’t function yet.


Third trimester (weeks 28-40)

Changes in the mother’s body

Due to the fetus weight, the enlarged uterus will push against your diaphragm, so you may feel shortness of breath, according to Johns Hopkins Medicine. This will also cause your face, hands, ankles, and feet, to swell as you retain more fluids and your blood circulation slows. You may notice some dark patches on your skin and stretch marks on your belly, thighs, breasts, and backside.

The back pain will be more intense in the hips and pelvis, as these joints relax in preparation for delivery.

During this period, according to the OWH, you may experience some leakage of a yellow liquid (colostrum) from your breasts, as they get ready for breastfeeding.

The closer you are to the due date, the more likely you will experience false labor, known as Braxton-Hicks contractions.


Third trimester

Development of the fetus

When the trimester arrives, your baby will be more active by kicking more and stretching. They can open and close their eyes, and can even respond to light and sound, like music, Burch said.

During the eighth month of pregnancy, the fetus will gain weight, its bones will harden, except for the skull which remains soft and flexible to make delivery easier.

According to ACOG, the fetus is even able to hiccup during this time.

By the ninth month, the fetus begins to get ready for delivery by turning into a head-down position in a woman’s pelvis. The fetus’ lungs are now completely mature to functioning on their own.

An uncomplicated, healthy pregnancy should last 39 to 40 weeks. However, pregnancy length may vary as there are times when the baby is born before 39 weeks of pregnancy, which is considered preterm, or after 40 weeks, which is considered overdue.

See below the period for each term, released in the journal Obstetrics & Gynecology:

Early Term – between 37 weeks and 38 weeks 6 days.

Full Term – between 39 weeks and 40 weeks 6 days.

Late Term – the 41st week.

Post Term – after 42 weeks.