Stages of Growth Month-by-Month in Pregnancy
The fetus will change a lot throughout a typical pregnancy. This time is divided into three stages, called trimesters. Each trimester is a set of about three months. Your healthcare provider will probably talk to you about fetal development in terms of weeks. So, if you’re three months pregnant, you’re about 12 weeks.
You’ll see distinct changes in the fetus, and yourself, during each trimester of pregnancy.
First trimester
The first trimester will span from conception to 12 weeks. This is generally the first three months of pregnancy. During this trimester, the fertilized egg will change from a small grouping of cells to a fetus that begins to have human features. The first trimester is exciting, but also when most people develop unpleasant symptoms like morning sickness and fatigue.
Month 1 (weeks 1 through 4)
Although it’s strange, the first two weeks of pregnancy are a “getting ready” period. Your body slowly releases more hormones and your uterus prepares for a potential pregnancy. At the end of the second week, your ovary releases an egg (ovulation). If sperm meets an egg just after ovulation, the process to pregnancy continues.
- Week 3: Fertilization occurs during the third week. A sperm and egg join and create a zygote.
- Week 4: The tiny bundle of cells turns into a blastocyst and implants into your uterine lining. The beginning of what will become the placenta forms. A water-tight sac forms around the blastocyst. This is the amniotic sac, and it provides cushioning to a fetus during pregnancy.
By the end of the fourth week, the blastocyst is about 2 millimeters (mm) long — the size of a poppy seed.
Month 2 (weeks 5 through 8)
The second month of pregnancy is when most people realize they’re pregnant. Pregnancy hormones go into overdrive, and by about the fifth week, an at-home pregnancy test will show as positive. This is when many people begin to feel symptoms of pregnancy.
- Week 5: The neural tube (brain, spinal cord and other neural tissue of the central nervous system) forms. The tiny “heart” tube will beat 110 times a minute by the end of the fifth week.
- Week 6: Tiny buds that become arms and legs also develop. Blood cells are taking shape, and circulation will begin. Structures that’ll become the ears, eyes and mouth take form. Your healthcare provider can probably detect a heartbeat on a vaginal ultrasound.
- Week 7: Bones begin replacing soft cartilage and genitals begin to form. The embryo’s head is large in proportion to the rest of its body. Some people think the embryo resembles a small tadpole or seahorse due to its prominent tail (which becomes legs) and large head.
- Week 8: All of the major organs and body systems are developing. The fetus has web-like hands and feet. Eyes become visible and ears begin to form. The umbilical cord is fully developed and helps to transport oxygen and blood to the embryo.
After the 8th week, healthcare providers refer to the embryo as a fetus. It will remain a fetus until birth.
By the end of the second month, the fetus is about 0.5 to 1 inch (in) long — about the size of a black bean.
Month 3 (weeks 9 through 12)
The third month of pregnancy is when an embryo becomes a fetus. It’s a period of rapid growth and development. The fetus develops distinct facial features, limbs, organs, bones and muscles. By the end of the 12th week, the fetus has an assigned sex, but it won’t be visible on ultrasound for several more weeks.
- Week 9: The beginnings of teeth and taste buds are forming. Its muscles are forming and its body shape takes on more of a human appearance. But, its head is still 50% of its length. Your provider may be able to hear its heartbeat with a Doppler ultrasound.
- Week 10: The arms, hands, fingers, feet and toes are fully formed (no more webbed fingers). Fingernails and toenails are beginning to develop and the external ears form. The external genitals also begin to form, but it’s too soon to see them on an ultrasound.
- Week 11: The fetus is starting to explore a bit by doing things like opening and closing its fists and mouth. Its knees, elbows and ankles are working, but it’s too soon to feel any kicks. The bones are hardening, but its skin is still see-through. Facial features are more prominent.
- Week 12: All the organs, limbs, bones and muscles are present and will continue to develop in order to become fully functional. The circulatory, digestive and urinary systems are also working and the liver produces bile. The fetus is drinking and peeing amniotic fluid.
Since the most critical development has taken place, your chance of miscarriage drops considerably after 12 weeks (the end of the first trimester). Most people begin feeling some relief from morning sickness now, too.
At the end of the third month, the fetus is about 2.5 to 3 inches long — about the size of a plum.
Second trimester
The second trimester of pregnancy is often thought of as the best part of the experience. By this time, any morning sickness is probably gone and the discomfort of early pregnancy has faded. You may also start to feel movement as the fetus flips and turns in your uterus. During this trimester, many people find out about the fetus’s assigned sex. This is typically done during an anatomy scan (an ultrasound that checks physical development) at around 20 weeks.
Month 4 (weeks 13 through 16)
Many people begin showing signs of being pregnant at this point in pregnancy, especially if you’ve been pregnant before. Your pregnancy care provider can hear the fetal heartbeat loud and clear on a Doppler ultrasound. The fetus can even suck its thumb, yawn, stretch and make faces.
- Week 13: Vocal cords form and the fetus’s large head begins to grow proportionate to its body.
- Week 14: The fetus’s skin starts to thicken and fine hair begins to grow. It can start bringing its fingers to its mouth and turn its head. External genitals are fully developed and fingerprints begin to form.
- Week 15: Some organs, like intestines and ears, are moving to their permanent location. The fetus still uses amniotic fluid to practice breathing, but its lungs are beginning to develop. The fetus begins to make more purposeful movements, like sucking its thumb or smiling.
- Week 16: The fetus has lips and its ears are developed enough that it can hear you talk. Even though its eyes are closed, the fetus can react to light by turning away from it.
By the end of the fourth month, the fetus is about 5 inches long and weighs about 4 ounces. For reference, that’s about as big as an avocado.
Month 5 (weeks 17 through 20)
By the end of the fifth month of pregnancy, most people begin to feel the fetus moving around. The first movements are called quickening and can feel like a flutter. If your pregnancy has been healthy to this point, you’ll finally get your first ultrasound. You may even get to find out the fetus’s assigned sex.
- Week 17: The fetus still has thin skin, but will start to put on fat. Its skin is covered with a whitish coating called vernix. This “cheesy” substance is thought to protect fetal skin from long-term exposure to amniotic fluid.
- Week 18: The fetus is covered in lanugo, a peach fuzz-like hair. It helps keep the fetus warm and provides another layer of protection. The fetus may have a sleep-wake cycle, and loud noises may wake the fetus if it’s asleep.
- Week 19: The fetus is getting stronger and most people begin to feel kicks and punches. The fetus also has its own unique set of fingerprints and can hiccup.
- Week 20: The fetus’s nails grow towards the end of its fingers. The area of the brain responsible for its five senses begins to develop.
By the end of the fifth month, the fetus is about 9 to 10 inches long and weighs about 1 pound.
Month 6 (weeks 21 through 24)
If you could look inside your uterus right now, you’d see that the fetus’s skin is reddish in color, wrinkled and veins are visible through translucent skin. In the sixth month of pregnancy, its eyelids begin to part and you may notice regular, jerky movements. The fetus responds to sounds by moving or increasing its pulse.
- Week 21: Limb movements are coordinated and frequent. The fetus has bone marrow that helps it produce blood cells.
- Week 22: The fetus’s grasp is getting stronger and it can touch its ears and the umbilical cord. It can hear your heartbeat, your stomach rumble and your breathing.
- Week 23: If born prematurely, the fetus may survive after the 23rd week with intensive care. It will begin rapidly adding fat to its body.
- Week 24: The fetus’s lungs are fully developed, but not well enough to work outside your uterus.
By the end of the sixth month, the fetus is about 12 inches long and weighs about 2 pounds.
Month 7 (weeks 25 through 28)
The fetus continues to mature and develop reserves of body fat. The fetus changes position frequently and responds to stimuli, including sound, pain and light. The amniotic fluid begins to diminish.
- Week 25: More body fat makes the fetus’s skin less wrinkled and plumper. Its nervous system is quickly maturing.
- Week 26: The fetus makes melanin, the substance that gives skin and eyes their color. The fetus’s lungs start to make surfactant, a substance that helps it breathe after birth.
- Week 27: The fetus can open its eyes and blink. It also has eyelashes.
- Week 28: The fetus may begin turning head-down in your uterus as it gets ready for birth. At the end of the seventh month, the fetus is about 14 to 15 inches long and weighs between 2 and 3 pounds.
Third trimester
This is the final part of your pregnancy. You may be tempted to start counting down the days to your due date and hope that it comes early, but each week of this final stage of development helps the fetus prepare for birth. Throughout the third trimester, the fetus gains weight quickly, adding body fat that’ll help after birth.
Your healthcare provider will monitor you closely as you approach your due date. You’ll visit your provider biweekly and then weekly. Make sure to ask your provider any questions you have about labor and delivery.
Month 8 (weeks 29 through 32)
The fetus continues to mature and develop reserves of body fat. The brain develops most rapidly during this time. The fetus can see and hear most stimuli. Most internal systems are well-developed, but the lungs may still be immature.
- Week 29: You may notice the kicks and jabs feel more like pokes now that the fetus is getting cramped in the amniotic sac.
- Week 30: The fetus can control its own body heat. Its brain is maturing and growing rapidly.
- Week 31: The fetus can process more information and stimuli. You can probably notice more distinct patterns in when it’s awake and when it’s asleep.
- Week 32: The fetus’s skin isn’t translucent anymore. Other than the lungs and brain, most other organs are well-formed and ready for birth.
The fetus is about 17 to 18 inches long and weighs as much as 5 pounds.
Month 9 (weeks 33 through 36)
During this stage, the fetus continues to grow and mature. The lungs are close to being fully developed at this point in pregnancy. The ninth month is mostly about putting the finishing touch on growth and brain development.
- Week 33: The fetus’s bones are hardening, with exception of its brain, which needs to be soft to descend the birth canal.
- Week 34: The vernix that protects the fetus’s skin starts to get thicker.
- Week 35: The fetus’s brain continues to grow, but still only weighs two-thirds of what it should at birth.
- Week 36: The fetus loses its lanugo and it has hair on its head.
The fetus is about 17 to 19 inches long and weighs from 6 to 7 pounds.
Month 10 (Weeks 37 through 40)
In this final month, you could go into labor at any time. At this point, the fetus’s position may have changed to prepare for birth. Ideally, it’s head-down in your uterus. You may feel very uncomfortable in this final stretch of time as the fetus drops down into your pelvis and prepares for birth. Your provider may encourage you to perform kick counts, which is a way to track how much the fetus moves.
- Week 37: The fetus’s toenails reach the end of its toes. You may start to feel the fetus drop into your pelvis.
- Week 38: The fetus is packing on 0.5 pounds per week to get to its final size.
- Week 39: The fetus is full-term and ready to meet the world!
- Week 40: It’s your due date week. Call your pregnancy care provider if you notice any signs of labor.
The fetus is about 18 to 20 inches long and weighs about 7 to 9 pounds.