Having a baby: Stages of pregnancy by trimester

Stages of pregnancy: A woman in the early stages of pregnancy looks at printed ultrasound images.
The stages of pregnancy are described in three trimesters, with a fourth trimester after birth. (Image credit: Getty Images/SDI Productions)

For a pregnant person, feeling a new life developing inside your body is an amazing experience, even though you may not always feel your best during all the stages of pregnancy. One of the biggest changes for most pregnant people is weight gain — a pregnant person carrying one baby should gain between 11 and 40 lbs. (5 to 18 kilograms) by the end of the pregnancy, depending on pre-pregnancy weight, according to the Centers for Disease Control and Prevention

But there are many other changes too, and exactly how pregnancy affects the body can be very different from person to person, and even for the same mother from one pregnancy to the next. Some symptoms of pregnancy last for several weeks or months, while other discomforts are temporary or don't affect everyone the same way. 

"Pregnancy is a long, 10-month journey," said Dr. Draion Burch, an obstetrician and gynecologist at Magee Womens Hospital at the University of Pittsburgh Medical Center. 

(If you are looking for a guide for how to get pregnant, Live Science has gathered tips to increase fertility.)

A normal pregnancy usually lasts about 40 weeks, counting from the first day of the last menstrual period, which is around two weeks before conception actually occurs. Pregnancy is divided into three trimesters, each lasting between 12 and 13 weeks. During each trimester, changes take place in a pregnant person's body as well as in the developing fetus.

Some medical experts suggest that mothers and doctors should also recognize a "fourth trimester," which is the 12-week period after birth in which babies are adjusting to life outside the womb and women are coping with motherhood and on-going changes in their bodies, according to a paper published in July 2017 in the American Journal of Obstetrics and Gynecology

Conception & implantation

About two weeks after a period, ovulation occurs, during which the ovaries release typically one egg, but sometimes two or more eggs. The egg or eggs can be fertilized by a sperm cell 12 to 24 hours after release, as the egg travels down the fallopian tube toward the uterus. 

The sex of the fetus is determined at the time of fertilization, or conception, and depends on whether the egg receives an X or Y chromosome from a sperm cell. If the egg receives an X chromosome, the baby will be a girl; a Y chromosome means the baby will be a boy. 

According to the Cleveland Clinic, it takes about three to four days for the fertilized egg (or embryo) to move to the lining of the uterus, where it attaches or implants to the uterine wall. Once the embryo is implanted, the cells start to grow, eventually becoming the fetus and the placenta, which is tissue that attaches to the lining of the uterus. The placenta transports oxygen, nutrients and hormones from the mother's blood to the developing fetus via the umbilical cord throughout pregnancy.

First trimester, weeks 1-12

First trimester changes in the mother:

Pregnant people will experience a lot of symptoms during the first trimester as the body adjusts to the hormonal changes of pregnancy. In the early weeks, the pregnancy may not be showing much on the outside of her body, but inside many changes are taking place. 

Related: Am I pregnant? 12 early signs of pregnancy

One of the first changes is the production of human chorionic gonadotropin (hCG) hormone, which shows up in the blood right after conception occurs. Levels of hCG can be detected in a pregnant person's urine about a week after a missed period. Human chorionic gonadotropin is what's detected in a positive home pregnancy test

Rising levels of hCG and other hormones, such as estrogen, may be responsible for the waves of nausea and vomiting known as morning sickness that's most common during the first few months of pregnancy. Despite its name, morning sickness can occur any time of day. 

A pregnant person may also feel more tired than usual during the first trimester, a symptom that's linked with rising levels of the hormone progesterone, which increases sleepiness. 

Early in pregnancy, breasts may feel more tender and swollen, another side effect of rising levels of pregnancy hormones. The areolas, the skin around each nipple, will darken and enlarge. 

A pregnant person's digestive system may slow down to increase the absorption of beneficial nutrients. But reduced mobility of the digestive system might also trigger such common complaints as heartburn, constipation, bloating and gas, according to the Office on Women's Health (OWH)

Many parts of the body will work harder during pregnancy, including the heart. Heart rate will increase to pump more blood to the uterus, which will supply it to the fetus. 

Besides the physical changes, expecting mothers may also experience emotional highs and lows in the early months of her pregnancy and throughout it. These emotions may range from weepiness, mood swings and forgetfulness, to fear, anxiety and excitement. 

Related: Mood swings & mommy brain: The emotional challenges of pregnancy

First trimester embryo development:

A developing baby is called an embryo from the moment conception takes place until the eighth week of pregnancy. 

During the first month of pregnancy the heart and lungs begin to develop, and the arms, legs, brain, spinal cord and nerves begin to form, according to the American College of Obstetricians and Gynecologists (ACOG)

The embryo will be about the size of a pea around one month into a pregnancy, Burch said. Around the second month of pregnancy, the embryo has grown to the size of a kidney bean. In addition, the ankles, wrists, fingers and eyelids form, bones appear, and the genitals and inner ear begin to develop. 

After the eighth week of pregnancy and until birth occurs, a developing baby is called a fetus.

By the end of the second month, most of the fetus' main organs will have formed, Burch said. At this stage of pregnancy, he stressed, it's extremely important that pregnant women do not take harmful medications, such as illegal drugs. The first trimester is also the period when most miscarriages and birth defects occur. Between 10% and 20% of known pregnancies end in miscarriages, mostly because the fetus is not developing normally, according to the Mayo Clinic. Studies have found the risk of miscarriage drops significantly after the 12th week of pregnancy.

As early as the 10th week of pregnancy, a pregnant person may opt to have a noninvasive prenatal test (NIPT). This testing uses a blood sample from the mother to analyze DNA from the fetus to help determine the risk of the baby being born with certain genetic abnormalities, according to MedlinePlus. The mother can also find out the sex of the baby at this time, also through providing a blood sample that analyzes fetal DNA. None of these tests come in contact with or cause any harm to the fetus

During the third month of pregnancy, the fetus' bones and muscles begin to grow, buds for future teeth appear, and fingers and toes grow. The intestines begin to form and the skin is almost transparent. 

Second trimester, weeks 13-27

Second trimester changes in the mother:

By the second trimester, some of the unpleasant effects of early pregnancy may lessen or disappear as the body adjusts to its changing hormone levels. Sleeping may get easier and energy levels may increase. 

Nausea and vomiting usually get better and go away, Burch told Live Science. But other symptoms may crop up as the fetus continues its growth and development. 

Pregnant people will start to feel more pelvic pressure, Burch said, adding that the pelvis feels heavy like something is weighing it down. 

A more visible baby bump appears as the uterus grows beyond the pelvis, and the skin over the expanding belly may itch as it stretches, according to the OWH.

As the fetus is getting bigger and the mother is gaining more pregnancy weight in the front of her body, she may also experience more back pain, Burch said. A 2020 study published in the Journal of Personalized Medicine found that while most pregnant people experienced back pain, it was worse in those who were not physically active.

Sometime between the 16th and 18th weeks of pregnancy, a first-time mother may feel the first fluttering movements of the fetus, known as quickening, Burch said. It it isn't the first pregnancy, the mother is likely to feel the fetus kicking, squirming or turning even sooner because she knows what to expect, he said. 

The 20th week usually marks the halfway point of pregnancy. From about 20 weeks, it is possible to develop preeclampsia, a condition characterized by high-blood pressure, according to the Mayo Clinic. If left untreated, the condition can lead to serious, even fatal, complications for mother and baby. Other symptoms include rapid weight gain and swelling.

Burch encourages his patients to take a "baby-moon" — a mini-vacation or weekend getaway — during the second trimester, and he said the best time to get away is around the 28th week of pregnancy. Expecting mothers are generally feeling pretty good at this point, and there's a lower risk of miscarriage and premature labor. A study published in 2020 in the journal PLOS One, found that for hundreds of thousands of babies, their mother's airplane travel had no adverse effects on the baby's birth weight or gestational age. However, some health professionals and airlines may discourage airplane travel after the 36th week because at that time birth could be just around the corner. 

Second trimester fetal development:

In the second trimester, the fetus will be between 3 and 5 inches (7 and 12 centimeters) long, Burch said. Sometime between 18 and 22 weeks, an ultrasound may reveal the sex of the baby, if parents want to know this information in advance. 

Related: Am I having a boy or girl? Ultrasound and sex prediction

By the fourth month of pregnancy the fetus' eyebrows, eyelashes, fingernails and the neck form, and the skin has a wrinkled appearance. In addition, during the fourth month the arms and legs can bend, the kidneys start working and can produce urine, and the fetus can swallow and hear, according to the ACOG

In the fifth month of pregnancy, the fetus is more active and the mother may be able to feel its movements. The fetus also sleeps and wakes on regular cycles. A fine hair (called lanugo) and a waxy coating (called vernix) cover and protect the thin fetal skin. 

By the sixth month of pregnancy, hair begins to grow, the eyes begin to open and the brain is rapidly developing. Although the lungs are completely formed, they don't yet function. 

Third trimester, weeks 28-40

Third trimester changes in the mother:

During the third trimester, as the mother's enlarged uterus pushes against her diaphragm, a major muscle involved in breathing. Because of this, she may feel short of breath because the lungs have less room to expand, according to Johns Hopkins Medicine. A pregnant person's ankles, hands, feet and face may swell due to fluid retention and slower blood circulation. 

A mother-to-be will need to urinate more frequently because of the pressure on her bladder. She may also have more backaches and more pain in the hips and pelvis, as these joints expand in preparation for delivery. Changes in body shape can also make a pregnant person more unstable on their feet and more likely to fall, according to a 2006 study published in the American Journal of Obstetrics and Gynecology.

Her face may develop dark patches of skin, and stretch marks may appear on her belly, thighs, breasts and backside. She may also notice varicose veins on her legs. 

In the third trimester, the breasts may start to leak colostrum, a yellow liquid, as they get ready for breastfeeding, according to the OWH. The baby will start to drop lower in the mother's abdomen. 

False labor, known as Braxton-Hicks contractions, may begin to occur as the due date gets closer. A "nesting instinct," behavior exhibited in many mammals, may kick in as expecting parents work on baby-proofing their home, shop for baby items, prepare the nursery and await their new arrival.

Related: Signs of labor: 6 clues baby is coming soon

During the final weeks of pregnancy, it will become harder to find a comfortable sleeping position, adding to the increased levels of fatigue, Burch said. 

Third trimester fetal development:

By the seventh month of pregnancy, the fetus kicks and stretches, and can even respond to light and sound, Burch said. Its eyes can also open and close. 

During the eighth and ninth months of pregnancy, the fetus gains weight very quickly. Bones harden, but the skull remains soft and flexible so the head can fit through the birth canal. Different regions of the brain are forming, and the fetus is able to hiccup, according to ACOG.

The ninth month is the home stretch of pregnancy, and the fetus is getting ready for birth by turning into a head-down position in the mother's pelvis. The lungs are now fully mature and ready to function on their own. 

The definition of a full-term pregnancy is when a baby is born after 39 to 40 weeks (it used to be 37 weeks), Burch said. This is because babies born at full-term had lower risks of problems with breathing, feeding and regulating their temperature than those born earlier, according to the National Institutes of Health.

Fourth trimester, post-birth

The period called the "fourth trimester" begins as soon as birth is over. While this can be a joyful and exciting time for most new parents, it can also be extremely challenging and stressful. 

In the weeks and months after the baby is born, the mother's body continues to go through major changes as it heals and recovers from pregnancy and birth. At the same time, the mother must grapple with the physical and emotional challenges of feeding and caring for a newborn, Dr. Ilona T. Goldfarb, a maternal health specialist, wrote for the Harvard Health Blog. "Women and their families experience substantial physiological, social and emotional changes," during this time, Goldfarb wrote. 

Despite the continuing changes and challenges that occur after childbirth, most mothers visit their obstetrician only once in the weeks after birth. Many health experts agree that this lack of attention to maternal health in the fourth trimester is concerning, especially considering that more than half of pregnancy-related deaths occur after the birth of the child, according to a 2018 report from the ACOG

Fortunately, several of these experts are putting pressure on the healthcare community to provide more comprehensive postpartum care. Goldfarb recommends pregnant people come up with a postpartum plan to help anticipate difficulties and how to deal with them as they pop up. She also recommends mothers speak with their obstetrician to learn more about the kind of postpartum resources they have. 

Additional resources


Pregnancy Day By Day: An Illustrated Daily Countdown to Motherhood, from Conception to Childbirth

This article is for informational purposes only, and is not meant to offer medical advice. This article was updated on June 14, 2021 by Live Science contributor Sarah Wild. 

Live Science Contributor

Cari Nierenberg has been writing about health and wellness topics for online news outlets and print publications for more than two decades. Her work has been published by Live Science, The Washington Post, WebMD, Scientific American, among others. She has a Bachelor of Science degree in nutrition from Cornell University and a Master of Science degree in Nutrition and Communication from Boston University.