Within 24 hours after fertilization, the egg that will become your baby rapidly divides into many cells. By the eighth week of pregnancy, the embryo develops into a fetus. There are about 40 weeks to a typical pregnancy. These weeks are divided into three trimesters.
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.
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.
The embryonic stage lasts from about the third week of pregnancy until 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 embryo’s heart begins to develop and pulse around the sixth week. 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.
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.
Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy
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.
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.
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.
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.
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.
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.
By the end of the fourth week, the blastocyst is about 2 millimeters (mm) long — the size of a poppy seed.
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.
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.
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.
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.
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.
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.
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.
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.
By the end of the fifth month, the fetus is about 9 to 10 inches long and weighs about 1 pound.
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.
By the end of the sixth month, the fetus is about 12 inches long and weighs about 2 pounds.
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.
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.
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.
The fetus is about 17 to 18 inches long and weighs as much as 5 pounds.
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.
The fetus is about 17 to 19 inches long and weighs from 6 to 7 pounds.
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.
The fetus is about 18 to 20 inches long and weighs about 7 to 9 pounds.
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.
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.
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.
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:
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.
Last reviewed by a Cleveland Clinic medical professional on 03/03/2023.
Learn more about our editorial process.