The evolution of pregnancy week by week is marked by different stages of fetal development. The first 12 weeks are considered the most important and the most sensitive, as it is a period of rapid cellular division during which the vital organs are forming.

Each stage of pregnancy comes with new developments, including changes to the mother's body that can cause a variety of uncomfortable symptoms such as morning sickness, fatigue, and breast tenderness.

At around 12 weeks of pregnancy, the placenta and umbilical cord will form, which will be responsible for transporting oxygen and nutrients necessary for fetal development. The fetal organs will continue to develop until the end of pregnancy, which typically lasts 9 months (or 40 weeks) in total. In some cases, pregnancy may continue up until 42 weeks.

pregnant person looking at ultrasound pictures

Month 1: Weeks 0-4 of pregnancy

The 1^st month of pregnancy corresponds to the first 4 weeks of gestation and is the beginning of the first trimester.

In this stage of pregnancy, the placenta has not yet formed, but the embryo is present in the gestational sac, which will protect it from infections and injury and will eventually be replaced by the placenta and the bag of waters when they develop at around 12 weeks.

Maternal changes: in this stage, implantation of the fertilized egg in the uterus stimulates the body to produce the hormone beta-HCG, which helps create favorable conditions for the embryo to develop. Increasing levels of beta-HCG can also lead to the first symptoms of pregnancy, such as morning sickness, fatigue, and breast tenderness, which are often confused with PMS symptoms.

Fetal changes: by the end of the first month, the embryo measures about 2 mm and the neural tube has formed that will give rise to the brain and nervous system.

Month 2: Weeks 5-8 of pregnancy

The 2^nd month of pregnancy corresponds to weeks 5 through 8 of gestation, which is when the majority of people discover they are pregnant.

Maternal changes: it is common for pregnant people to develop symptoms during this time, such as morning sickness and general malaise, caused by rapid changes in hormone levels. These symptoms will typically last until about the end of the third month of pregnancy. Some tips for managing these symptoms include avoiding strong smells and heavy meals. Nausea can be worse on an empty stomach, so eating small, frequent meals can also help. Adequate rest can also help combat fatigue, which can make morning sickness worse.

Fetal changes: at the beginning of the second month, the heart has already formed and started to beat and pump blood at an accelerated rate. By the end of this month, other organs like the lungs, liver, intestines, and kidneys will also start to form, as well as the brain cells and the small limb buds that will give rise to the arms and legs.

At the end of the second month, the embryo is still very small, measuring around 25 mm (1 inch).

Month 3: Weeks 9-13 of pregnancy

The 3^rd month of pregnancy corresponds to weeks 9 through 13 of gestation, and is also considered the end of the first trimester.

Maternal changes: a small "baby bump" may start to appear at this time, however it is still normal to not look pregnant at all. Morning sickness is usually less frequent by this time and the breasts have started to grow in volume. This rapid growth can result in stretch marks, which are often genetic.

Fetal changes: this month marks the formation of the bones, cartilage, ear canals, nostrils, and elbows. In fact, you may even see the fetus flexing its arms during an ultrasound. The fetal organs have formed and are beginning to function, however they will continue to develop and mature up until the end of pregnancy. In addition, the ovaries or the testicles are also completely formed by now.

At the end of the third month, the fetus measures about 7.4 cm (3 inches) and the placenta is completely formed. If you opt for genetic testing, a a blood test called NIPT (noninvasive prenatal testing) can be done to check for the risk of certain genetic disorders in the fetus, such as Down syndrome. It can also determine fetal sex.

Month 4: Weeks 14-17 of pregnancy

The 4^th month of pregnancy corresponds to weeks 14 through 17 of gestation, and is the start of the second trimester.

Maternal changes: a pregnant person may start to "show" by this stage, with the waist appearing rounder and a "baby bump" becoming more obvious.

Fetal changes: the fetus's thin skin is covered by a fine layer of hair called lanugo, which helps to regulate body temperature. While the eyelids are still sealed shut, the fetus can already move its eyes back and forth and appreciate the difference between light and dark.

During this month, the fetus begins to swallow amniotic fluid and starts developing the suck/swallow reflex that will be needed for feeding. The lungs are continuing to develop and the fetus is engaging in "practice breathing". These breathing-like movements involve the "inhalation" (or swallowing) of amniotic fluid.

By the end of the fourth month the fetus measures around 19.6 cm (7.5 inches) from head to toe and weighs in around 179 grams (6 ounces).

Month 5: Weeks 18-22 of pregnancy

The 5^th month of pregnancy corresponds to weeks 18 through 22 of gestation.

Maternal changes: a dark line known as the linea nigra may appear, extending from the navel (bellybutton) to the pubic bone. This is normal and occurs as a result of hormone changes during pregnancy. Braxton-Hicks, or "practice contractions", may also start during this time, which are mild and considered a normal part of pregnancy.

Fetal changes: in this phase, the ears are already well formed, allowing the fetus to hear the mother's heartbeat. In addition, if the fetus is female, the fallopian tubes are already in the correct position. It is now possible to confirm fetal sex on a 20 week ultrasound, or anatomy scan, that usually takes place between 18 and 22 weeks of pregnancy.

The vernix caseosa is starting to form, which is a white, cheesy substance covering the fetus that keeps the skin hydrated and helps maintain body temperature and prevent infection.

By the end of the fifth month of pregnancy, the fetus will measure about 25 cm (10 inches) and weigh around 400 grams (14 ounces). Most pregnant people will also begin to feel fetal movement by this time.

Month 6: Weeks 23-27 of pregnancy

The 6^th month of pregnancy corresponds to weeks 23 through 27 of gestation and marks the end of the second trimester.

Maternal changes: it is usually easier to feel fetal movements by this time, and you may find that stroking your belly and talking to the baby can help to calm them.

Fetal changes: the fetus will open its eyes this month and already has a sleep routine and a more developed palate. Hearing becomes increasingly clear and the fetus is now able to appreciate external stimuli, perhaps responding to touch or being startled by loud noises.

By the end of this month, the fetus will measure around 35 cm (13.5 inches) from head to toe and weigh about 1000 grams (2 pounds 3 ounces).

Month 7: Weeks 28-31 of pregnancy

The 7^th month of pregnancy corresponds to weeks 28 through 31 of gestation, and marks the beginning of the third trimester.

Maternal changes: the pregnant person may start to experience back pain or pelvic pain related to changes in the curvature of the spine as well as the loosening of the joints that happens in preparation for childbirth.

Fetal changes: in this phase, the fetal head is continuing to grow and the brain is developing and expanding. The nutritional needs of the fetus are also increasing. Fetal movements are becoming stronger and more frequent, with the fetus recognizing the mother's voice and reacting to changes in light and sound.

By the end of the seventh month, the fetus measures around 39.4 cm (15.5 inches) and weighs about 1800 grams (4 pounds). In this stage, it is important to start preparing for the baby's arrival. Make sure to start packing your "hospital bag" and have baby clothes and a crib ready.

Month 8: Weeks 32-35 of pregnancy

The 8^th month of pregnancy corresponds to weeks 32 through 35 of gestation.

Maternal changes: for the pregnant person, this stage can be uncomfortable due to swollen feet and the appearance of new or worsening varicose veins. Taking walks and staying active while allowing for plenty of rest can help with these symptoms.

Fetal changes: the fetal lungs are more mature at this stage, although they will continue to develop up until the end of pregnancy. The lungs are also producing surfactant, a substance that will help with oxygen transfer during breathing after birth.

By the end of the eighth month of pregnancy, the fetus measures about 45.7 cm (18 inches) and weighs around 2700 grams (6 pounds). In this stage, the fetus has already begun to move its head side to side and there is less space to move around. The digestive system is fully developed and the bones are becoming stronger.

Month 9: Weeks 36-42 of pregnancy

The 9^th month of pregnancy corresponds to weeks 36 through 40 of pregnancy, but may extend to 42 weeks.

Maternal changes: pregnant people may find it increasingly difficult to sleep this month due to the size of the growing belly. Other common symptoms of this stage include more frequent Braxton-Hicks contractions, heartburn, and swollen feet and ankles that are usually worse at the end of the day.

Fetal changes: by the 37^th week of pregnancy, most fetuses will already be head down. Some fetuses may take a bit longer to turn into the head down position, but position should be confirmed around this time in order to make plans for delivery.

By 40 weeks of pregnancy, the fetus will measure about 51 cm (20 inches) and weigh around 3600 grams (8 pounds) and is considered to be full term. Post-term begins at 42 weeks, meaning a baby can be born up until 41 weeks 6 days and not be considered overdue.

If labor does not start on its own by 41 weeks 0 days, your OBGYN or midwife may recommend an induction of labor. This usually involves administering oxytocin through an IV to stimulate contractions, which takes place in the hospital. Your provider may recommend an induction sooner than this based on your own individual risk factors.