Week-to-Week Pregnancy Timeline
A Detailed Look at All the Changes That Take Place
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Medically reviewed by Cordelia Nwankwo, MD
In pregnancy, a fertilized ovum implants in the uterus and develops into an embryo and fetus before being born. The pregnant person will experience changes throughout pregnancy, beginning with missing a menstrual period.
Physical changes and symptoms during pregnancy include menstrual period absence, morning sickness, breast changes, weight gain, change in body shape, feeling fetal movement, and more.
Pregnancy is divided into three trimesters. The first trimester lasts from week one to week 12. The second lasts from week 13 to week 26, and the third lasts from week 27 to the end of pregnancy.
Your healthcare provider will help you determine your due date, the date when you are most likely considered full-term and ready to give birth. The due date is calculated 40 weeks after the first day of your last period.
This article will provide an overview of pregnancy, focusing on how it feels and what to expect.
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When Does Pregnancy Start, and How Do You Know?
Knowing exactly when your pregnancy starts is not as straightforward as you’d imagine. Many people refer to “how many weeks they are” to keep track of their pregnancy. This is based on the fetus’s gestational age. Gestational age starts on the first day of your last period before conception.
The Menstrual Cycle
To understand the beginning of pregnancy, it’s important to start with the menstrual cycle. The average length of a menstrual cycle is 28 days but can range from 24 to 38 days. A person with a 28-day menstrual cycle usually ovulates around day 14.
The first two weeks of your pregnancy include your menstrual period and fertilization. About two weeks into your menstrual cycle, you will experience ovulation (the release of an egg from the ovary). The timing can vary depending on the length of your menstrual cycle. The average day to ovulate is day 14.
During ovulation, your ovaries release their most mature egg, which travels down the fallopian tube. This is where the egg meets the sperm and is fertilized (if pregnancy occurs).
During weeks three and four of your pregnancy, the fertilized egg travels to the uterus and begins dividing into more cells. It usually reaches the uterus about three to four days after fertilization.
Once this cluster of cells implants into the uterine wall (known as implantation), you are officially pregnant. Implantation occurs about six days after fertilization and takes three to four days to complete.
Most people do not know that they are pregnant in these early weeks because there are usually no signs. The first sign of pregnancy is often a missed menstrual period. Once you notice that you did not get your period, it’s time to take a pregnancy test.
A pregnancy test measures the amount of human chorionic gonadotropin (hCG), a hormone, in your urine to detect pregnancy. Your body starts producing hCG after implantation, and the level doubles every 48 hours in the first few weeks of pregnancy.
If you take a test too early, your body may not have enough hCG to produce a positive result. When used correctly, home pregnancy tests are 99% accurate.
Prenatal care is the medical care that you receive from the time you find out you are pregnant until the pregnancy ends. Prenatal care includes the healthcare appointments and medical tests you will undergo throughout pregnancy. Participating in prenatal care is an essential way to take care of the health of both you and your baby.
Related: When and Why Should You Take Prenatal Vitamins?
Beginning Pregnancy Stages: First Trimester
The first trimester is a time of rapid change for both you and the embryo. You may not look very different on the outside, but your body is hard at work. The first trimester includes conception, implantation, and embryo development.
The early pregnancy symptoms you may experience during the first trimester include:
Spotting: Light bleeding or spotting is sometimes an early sign of pregnancy. About 15% to 25% of pregnant people experience bleeding in the first trimester. This is likely due to implantation bleeding. This occurs about six to 12 days after conception. If you experience heavy bleeding, see your healthcare provider right away.
Breast tenderness: About one to two weeks after conception, you may notice swollen, tender breasts. This happens because of an increase in the level of the hormone estrogen in early pregnancy.
Frequent urination: A rapid increase in estrogen levels can also cause you to feel the urge to urinate more. You will likely notice this in the first month of pregnancy.
Fatigue: Pregnancy requires a lot of energy, which can be draining for your body. It is common to experience fatigue during the first few months of pregnancy because of changing hormone levels.
Nausea: Morning sickness refers to nausea and sometimes vomiting during pregnancy. This usually starts about two to eight weeks after conception and is caused by increased hCG levels.
Bloating: During your first month of pregnancy, your level of the hormone progesterone begins to increase. This change can lead to bloating and constipation because it slows down the gastrointestinal (GI) tract.
Headaches: It is common to experience headaches during the first few weeks of pregnancy. The change in hormone levels likely causes them. If you notice very painful headaches or if they wake you up at night, talk with a healthcare provider immediately.
Middle Pregnancy Stages: Second Trimester
During the second trimester, your baby officially becomes known as a fetus rather than an embryo. The second trimester is the time when many people begin to “show” and develop a pregnant belly.
Many pregnant people begin feeling fetal movement around week 20. This often feels like light flutters (as opposed to strong kicks). By the 24th week of pregnancy, your fetus is beginning to sleep and wake at regular intervals. Loud noises may even wake them up.
Most pregnant people have an ultrasound exam between 18 and 20 weeks of pregnancy. During this exam, an ultrasound tech or sonographer will use the ultrasound machine to visualize your fetus and check for any anatomic anomalies.
The second trimester is often a time of improved symptoms. You may notice that you suddenly feel less tired and nauseous. It’s common for new symptoms to begin as well.
Possible second-trimester symptoms include:
Heartburn: Because progesterone slows down your digestive tract, it is common to experience heartburn or reflux during pregnancy.
Constipation: Constipation and bloating often continue in the second trimester.
Breast changes: Your breasts will likely continue to grow and feel very sensitive. The average pregnant person goes up two cup sizes by the fifth month of pregnancy. Your breasts may also begin to produce early breast milk known as colostrum.
Shortness of breath: During the second trimester, you may start to experience shortness of breath with activities such as walking up stairs. Shortness of breath at rest should be evaluated by your healthcare provider.
Nosebleeds: During pregnancy, your body experiences increased blood flow. This can lead to nosebleeds, as well as bleeding gums, nasal stuffiness, dizziness, and fainting.
Braxton-Hicks contractions: Braxton-Hicks contractions are painless, squeezing sensations in the lower abdomen. They are the uterus’s way of getting ready for labor.
Learn More: How Do Braxton-Hicks vs. Contractions Feel?
Late Pregnancy Stages: Third Trimester
As you enter your third trimester, your fetus’s body is getting ready for life outside the womb. Their bones are soft and almost fully formed by week 32. In the third trimester, your fetus begins gaining about one-half pound each week and moves into a head-down position.
Babies born between 37 and 38 weeks are considered early term. Babies born at 39 or 40 weeks are full term, and babies born at 41 weeks are late term. Babies born after 42 weeks are considered post-term.
In the third trimester, many of your symptoms continue. These include fatigue, shortness of breath, and Braxton-Hicks contractions. New symptoms you may notice include:
Back pain: It is common to experience back pain starting in the seventh month. This is due to the uterus expanding in size.
Varicose veins: Varicose veins are swollen superficial veins in the legs. Varicose veins may also form in the rectum and are known as hemorrhoids. This is due to the increased blood flow during pregnancy.
Stretch marks: Stretch marks are lines on the skin that form when the skin is overstretched from the weight gain in pregnancy.
Leaking urine: It is common to leak urine in your third trimester. This often happens when coughing or sneezing because the uterus is pressing down on the bladder.
Hair changes: The hormonal changes during pregnancy cause your hair to grow faster. It likely feels thicker and healthier as well.
Trouble sleeping: By the end of your third trimester, it may be difficult to fall asleep and stay asleep because you feel physically uncomfortable.
Pelvic pain: Many pregnant people experience sharp pelvic pain that comes and goes as the cervix dilates in the third trimester. Talk with your healthcare provider if you’re experiencing new-onset pain.
Heartburn and constipation may improve once your baby “drops”—moves into the lower part of the pelvis—near the end of the third trimester.
Infertility
If you have been experiencing infertility, you’re not alone. In the United States, about 10% of females between the ages of 15 and 44 experience difficulty getting pregnant. Infertility is defined as not becoming pregnant after trying for one year. (The terms for sex or gender from the cited source are used.)
There are several factors that affect your fertility, including stress, weight, diet, smoking, alcohol use, health conditions, and medications. If you are experiencing infertility, talk with a healthcare provider. If you are over 35, talk with a healthcare provider if you have been trying to become pregnant for more than six months.
Pregnancy Health Goals
Taking care of your health during your pregnancy can quickly begin to feel like a full-time job. There is so much to remember each day, and the care can start to feel overwhelming.
The most important step you can take to care for your pregnant body and baby is to connect with a healthcare provider. Receiving regular prenatal care reduces the risk of pregnancy complications.
Goals for your prenatal care may include:
Start seeing a healthcare provider such as an obstetrician-gynecologist (ob-gyn; a medical doctor who specializes in the care of pregnant people and delivering babies) or a certified nurse midwife (an advanced-practice nurse who specializes in pregnancy and postpartum care).
Choose a location to give birth. Options include the hospital, a birthing center, or your home.
Attend regular prenatal care appointments monthly for weeks four through 28, twice monthly for weeks 28 to 36, and weekly from week 36 to birth. Your provider may recommend more-frequent appointments if your pregnancy is considered high-risk.
Undergo prenatal blood tests to determine your blood type, Rh factor, anemia status, and if you have any infections.
Discuss your family history with your provider and meet with a genetic counselor if needed.
Have an ultrasound exam at 20 weeks and more frequently if needed.
Receive the recommended vaccinations, including for the flu and COVID-19. Your baby receives passive immunity to these diseases when you are immunized.
Continue to manage any chronic conditions you have, such as diabetes or arthritis.
Take care of your emotional and mental health by talking with your provider about stress management techniques and any concerns you have.
If you require financial assistance with prenatal care, ask your provider. Each state helps pay for medical care during pregnancy through financial aid programs.
There are also actions that you can take on your own. Lifestyle goals during pregnancy include:
Avoid alcohol, drugs, tobacco, and cannabis.
Eat a nutritious diet with plenty of vegetables, fruits, whole grains, healthy fats, and lean proteins.
Take a folic acid supplement (400 micrograms).
Avoid dangerous workplace exposures, if possible.
Talk with your healthcare provider about how you plan to feed your baby.
Obtain a new car seat and learn how to install it so it is safe.
Disparities for Black Pregnant People
It would be impossible to discuss prenatal health goals without addressing the inequities that exist within the healthcare setting. In the United States, Black pregnant people are 3 times more likely to die from a pregnancy-related cause than White pregnant people. About 80% of pregnancy-related deaths in the United States are preventable.
The factors involved in the health disparities affecting Black pregnant people (and those of other racial and ethnic groups) vs. non-Hispanic Whites include:
Socioeconomic factors: They experience a higher poverty rate.
Prepregnancy care: They tend to experience higher rates of obesity, hypertension, diabetes, and chronic illness.
Prenatal care: They may be less likely to receive quality prenatal care.
Structural racism and implicit bias: They experience discrimination in the healthcare system.
Summary
Pregnancy is an overwhelming time full of physical and emotional changes. You’re likely feeling a range of emotions, and that is normal. Because of the biological changes that occur during pregnancy, you also will experience many physical symptoms, and they will likely change as your pregnancy progresses.
Once you learn that you are pregnant, it is crucial to find a healthcare provider such as an ob-gyn or certified nurse midwife. Getting regular prenatal care is one of the most important ways you can take care of yourself and your baby during pregnancy.
