Fetal Development

What are the three stages of fetal development called?

Fetal development is an orderly and intricate process. It begins before you even know you’re pregnant and ends with the birth of your baby. Between conception and delivery, there are many detailed steps that have to occur.

There are three stages of fetal development: germinal, embryonic and fetal. Most people don’t talk about their pregnancy in these terms, but it can be helpful to know.

Germinal stage

The germinal stage is the shortest stage of fetal development. It begins at conception when a sperm and egg join in your fallopian tube. The sperm fertilizes the egg and creates a zygote. The zygote begins its journey down to your uterus over the course of about one week. During this journey, the zygote divides many times, eventually creating two separate structures. One structure eventually becomes the embryo (and later, the fetus) and the other becomes the placenta. Cell division continues at a rapid pace. Eventually, the zygote turns into a blastocyst. The blastocyst arrives at your uterus and implants into your uterine lining. If implantation is successful, your body immediately begins producing hormones to support a pregnancy. This also stops your menstrual period.

Embryonic stage

The embryonic stage lasts from about the third week of pregnancy until the end of the eighth week of pregnancy. The blastocyst begins to take on distinct human characteristics. It’s now called an embryo. Structures and organs like the neural tube (which later becomes the brain and spinal cord), head, eyes, mouth and limbs form. The cells that will form the fetal heart begin to cluster around five to six weeks and can pulse. Buds that will become arms and legs also form around the sixth week. By the end of the eighth week, most of the embryo’s organs and systems take shape. For a lot of people, this is the point in pregnancy where morning sickness begins.

Fetal stage

The fetal stage of development begins around the ninth week and lasts until birth. This is when the embryo officially turns into a fetus. The fetus gets its assigned sex around nine weeks of pregnancy, although your healthcare provider can’t detect it on ultrasound yet. The fetus’s major organs and body systems continue to grow and mature. Things like fingernails, eyelashes and hair also grow. The fetus is able to move its limbs, although you may not feel it until 20 weeks of pregnancy. The majority of growth — in both weight and length — happens in the fetal stage.

When does a pregnancy start?

The start of pregnancy is actually the first day of your last menstrual period (LMP). This is the gestational age of the fetus. It’s about two weeks ahead of when conception actually occurs. Though it may seem strange, the date of the first day of your last period will be an important date when determining your due date. Your healthcare provider will ask you about this date and will use it to figure out how far along you are in your pregnancy.

How does conception work?

Each month, your body goes through a reproductive cycle that can end in one of two ways. You’ll either have a menstrual period or become pregnant. This cycle is continuously happening during your reproductive years — from starting your period to menopause around age 50.

In a cycle that ends with pregnancy, there are several steps. First, a group of eggs (called oocytes) gets ready to leave your ovary for ovulation (release of the egg). The eggs develop in small, fluid-filled cysts called follicles. Think of these follicles as small containers for each immature egg. Out of this group of eggs, one will become mature and continue through the cycle.

The mature follicle now opens and releases the egg from your ovary. This is ovulation.

After ovulation, the opened follicle develops into a structure called the corpus luteum. This releases the hormones progesterone and estrogen. Progesterone helps prepare your uterine lining for pregnancy. If you don’t become pregnant during a cycle, this lining is what your body sheds during your period. If sperm fertilizes the egg, conception occurs and the fertilized egg begins its journey to your uterus, where it will implant.

How long is a pregnancy?

Traditionally, we think of pregnancy as a nine-month process. However, this isn’t always the case. A full-term pregnancy is 40 weeks, or 280 days. Depending on what months you’re pregnant during (some are shorter and some longer) and what week you deliver, you could be pregnant for either nine months or 10 months. This is completely normal and healthy.

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 pulse 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 pulses in the cluster of cells that will form the heart 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 recedes) and large head.
• Week 8: All of the major organs and body systems are developing. The embryo 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 the cranial bones around 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.

How early can I know I’m pregnant?

From the moment of conception, the hormone human chorionic gonadotrophin (HCG) will be present in your blood. This hormone is created by the cells that form the placenta (food source for the growing fetus). It’s also the hormone detected in a pregnancy test. Even though this hormone is there from the beginning, it takes time for it to build within your body. It typically takes three to four weeks from the first day of your last period for the HCG to increase enough to be detected by pregnancy tests.

When should I reach out to my healthcare provider about a new pregnancy?

Most healthcare providers will have you wait to come in for an appointment until you’ve had a positive home pregnancy test. These tests are very accurate once you have enough HCG circulating throughout your body. This can be a few weeks after conception. It’s best to call your healthcare provider once you have a positive pregnancy test to schedule your first appointment.

When you call, your healthcare provider may ask you if you’re taking a prenatal vitamin. These supplements contain folic acid. It’s important that you get at least 400 mcg of folic acid each day during pregnancy to make sure the fetus’s neural tube (beginning of the brain and spine) develops correctly. Many healthcare providers suggest that you take prenatal vitamins with folic acid even when you aren’t pregnant. If you weren’t taking prenatal vitamins before your pregnancy, your provider may ask you to start as soon as possible.

What weeks are most important for fetal development?

Each trimester or phase of pregnancy carries its own unique risks. It’s hard to pinpoint specific weeks as being more or less important. However, most healthcare providers will say the most important time for fetal development is the first 13 weeks of the pregnancy (or the first trimester). The risk of miscarriage drops after the first trimester, when the fetus’s major organs and systems are formed and working. This isn’t to say that your pregnancy becomes less important after 13 weeks. It just means a lot of the critical steps and processes are complete and that a lot of the most major birth disorders occur in the first trimester.

What does full-term mean in a pregnancy?

Once you get close to the end of your pregnancy, healthcare providers may use several terms to describe when you go into labor. These terms are labels that divide up the last few weeks of pregnancy.

They’re helpful in determining how likely a baby is to have complications at birth. For example, babies that are born in the early term period (or before it) generally have a higher risk of breathing issues than babies born at full term.

When you’re looking at these labels, it’s important to know how they’re written. You may see the week first (38) and then two numbers separated by a slash mark (6/7). This stands for how many days you currently are in the gestational week. So, if you see 38 6/7, it means that you’re on day 6 of your 38th week.

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.

Talk to your healthcare provider about any questions you may have about gestational age and due date.

A note from Cleveland Clinic

There’s a lot that needs to happen for a pregnancy to occur, grow and result in a birth. Learning about how the fetus grows can be exciting and eye-opening, especially when you realize how many organs, systems and body functions develop within a nine-month period. Both you and the fetus growing inside of you go through many changes during pregnancy. Ask your pregnancy care provider about these changes and any other questions you have. They’re there to be a resource to you and give you the best possible care.