During pregnancy, milestones matter a whole lot. They’re how you can keep tabs on the stages of prenatal development, find out if any potential issues arise, and know when and how to plan for birth and the postpartum period.

Many of the major milestones of pregnancy happen on a weekly or trimester basis. A “full-term” pregnancy lasts between 39 weeks to 40 weeks and 6 days. Births are considered “preterm” if they happen before 37 weeks of pregnancy. (1)

This is where things get confusing (but also offer up good trivia fodder): Because there’s no way to know for certain exactly when conception happens, experts assume that pregnancies begin two weeks after the last menstrual period. That means that two weeks are counted before you’re even technically pregnant. So, by the earliest time pregnancy tests can tell you you’re pregnant—as soon as two weeks after ovulation—you’re already considered four weeks pregnant! (2)

Now, let’s talk about trimesters (aka the three stages of pregnancy):
• The first trimester starts from the first day of your last menstrual period through 13 weeks and 6 days of pregnancy.
• The second trimester goes from 14 weeks through 27 weeks and 6 days of pregnancy.
• The third trimester covers 28 weeks through 40 weeks and 6 days of pregnancy.

With the overview out of the way, we can dive right into the specifics. Keep reading for what to know about expected pregnancy progress—including body changes and fetal development by trimester—so you’re up to speed on the major milestones.

Fetal Stage 1: First Trimester (Week 1-Week 12)

The first trimester can be a complex period wrapped up in excitement, worry, body changes, and “morning” sickness (it’s really more of an all-day thing)—among so many other feelings and emotions. And for at least one-third of the trimester, many people won’t even know they’re pregnant!

It’s for all of those reasons that knowing what to expect, both internally and externally, can be really helpful. The TL;DR? The first trimester is all about fertilization and organ development.

How Your Body May Change

According to the American College of Obstetricians and Gynecologists (ACOG), which is the leading national organization of OB/GYNs, these are some of the possible body changes to expect in the first trimester (2):
• No more periods
• Larger and more tender breasts/chest
• Nipples that stick out more
• More frequent urination
• Tiredness
• Nausea and vomiting
• Food cravings or loss of appetite
• Heartburn or indigestion
• Bloating and gas
• Constipation
• A few pounds of weight gain or loss

How the Pregnancy Develops

Let’s begin with what happens in the weeks leading up to a pregnancy:
• About two weeks before a pregnancy test can show a positive result, an
ovary releases an egg so it can be fertilized by sperm. (2)
• Sperm in the reproductive tract fertilize the egg within 24 hours of ovulation (that’s how long the egg can live). (3) The fertilized egg travels through the fallopian tube to the uterus.
• 1-2 weeks after ovulation, the fertilized egg implants itself into the lining of the uterus. (4) The cells surrounding the implanted now-embryo begin producing human chorionic gonadotropin (hCG). These cells eventually form into the placenta.
• Home pregnancy tests are most accurate at detecting hCG in the urine 1-2 weeks after a missed menstrual period. (5) However, some tests may detect hCG as soon as ~12-15 days post-ovulation. (6)

ACOG breaks down embryo and fetal development by week. Here’s what you need to know about embryo development in weeks 1-8:
• The embryo’s brain and spine (neural tube) begin to form.
• The embryo’s cardiac tissue, which will become the heart, starts to develop.
• The embryo’s eyes, nose, and mouth muscles form.
• The embryo’s developing hands and feet begin to show webbed fingers and toes.
• The embryo’s urinary system begins to develop. (10)
• The embryo’s lungs begin forming the tubes that will eventually carry air in and out.
• The embryo’s inner ear starts to develop.

The formation of the baby’s neural tube is why prenatal multivitamins with folate are recommended at least one month before getting pregnant. (11) Choline supports baby's neural tube development during pregnancy.* The fetal neural tube begins to form during weeks 3 and 4—before most people even realize they’re pregnant. (12)

What you need to know about fetal development in weeks 9-12:
• In the fetal stage as of week 10, the fetus's limbs, hands, and feet cartilage begin to form.
• The fetus's eyelids form but stay closed.
• The fetus's genitals and liver begin to develop.
• The fetus's kidneys and pancreas start producing urine and insulin.
• The fetus's fingernails form.

Fetal Stage 2: Second Trimester (Week 13-Week 28)

The second trimester is often when pregnant people feel a lot more comfortable, symptom-wise. It’s also when the body begins changing more visibly in response to rapid fetal growth and development.

How Your Body May Change

These are some of the possible body changes to expect in the second trimester:
• Increased appetite
• Decreased nausea and fatigue
• A bigger abdomen, tight and itchy skin, and often stretch marks
• Fetal movements will be noticeable
• Brown patches on your face
• Darkened areolas (the skin around the nipples)
• Swollen feet and ankles

How the Pregnancy Develops

What you need to know about fetal development in weeks 13-16:
• The fetus's bones harden.
• The fetus's skin begins to thicken.
• The fetus's toenails form.
• The fetus's neck becomes defined and lower limbs develop.
• The fetus begins to develop hearing.
• The fetus's lungs start forming tissue for breathing after birth.

What you need to know about fetal development in weeks 17-20:
• The fetus's brain center for motor movement control is fully formed.
• The fetus's digestive system now works.
• The fetus's ears, nose, and lips are recognizable via ultrasound.
• The fetus's uterus and vagina begin to form if they’re assigned female.
• The fetus develops soft, downy hair (lanugo) all over the body.

What you need to know about fetal development in weeks 21-24:
• The fetus's kicks and turns become stronger.
• The fetus's sucking reflex develops.
• The fetus forms fat under the skin.
• The fetus forms ridges in the hands and feet that will become fingerprints and footprints.
• The fetus's skin is wrinkled and reddish because blood vessels are visible.

What you need to know about fetal development in weeks 25-28:
• The fetus's lungs become fully formed but can’t function yet outside of the uterus.
• The fetus can respond to loud noises and startled movements.
• The fetus's eyelids open and close.
• The fetus's lungs start producing the substance surfactant, which is needed for breathing after birth.
• The fetus's nervous system begins developing.
• The fetus's skin looks smoother with more added fat.

Fetal Stage 3: Third Trimester (Week 29-Birth)

The third trimester is all about preparing for birth. Your body and the fetus are getting everything in place and in order to help bring a healthy baby into the world.

How Your Body May Change

These are some of the possible body changes to expect in the second trimester:
• More strongly felt fetal movements
• Shortness of breath
• More frequent urination from extra pressure on the bladder
• Nipples leak the yellow, watery colostrum (pre-milk)
• Belly button sticks out
• Practice or “true” contractions

How the Pregnancy Develops

What you need to know about fetal development in weeks 29-32:
• The fetus starts stretching, kicking, and making grasping motions.
• The fetus's eyes can detect changes in light.
• The fetus's bone marrow forms red blood cells.
• The fetus's head may have a little hair.
• The fetus's testicles have started descending from the scrotum if they’re assigned male.
• The fetus's lanugo starts to disappear.

What you need to know about development in weeks 33-36:
• The fetus's bones harden, but the skull stays soft and flexible.
• The fetus's fingernails have grown long enough to reach the ends of the fingers.
• The fetus's limbs start looking chubby.
• The fetus may get into a heads-down position in preparation for birth.

What you need to know about development in weeks 37-40:
• The fetus's circulatory and musculoskeletal systems are fully developed.
• The fetus's lungs, brain, and nervous system are reaching the end of development.
• The fetus continues putting on fat to keep them warm after birth.

Ritual’s Essential Prenatal Multivitamin is a clean and Made Traceable® prenatal multivitamin with key nutrients to support pre-pregnancy and pregnancy nutrient needs.

References:

1. American College of Obstetricians and Gynecologists’ Committee on Obstetric Practice. (2013). ACOG Committee Opinion No 579: Definition of Term Pregnancy. Obstetrics and Gynecology. Retrieved June 1, 2023, from https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2013/11/definition-of-term-pregnancy
2. American College of Obstetricians and Gynecologists. (2020). FAQs: How Your Fetus Grows During Pregnancy. American College of Obstetricians and Gynecologists. Retrieved June 1, 2023, from https://www.acog.org/womens-health/faqs/how-your-fetus-grows-during-pregnancy
3. American College of Obstetricians and Gynecologists. (2020). Infographic: Changes During Pregnancy. American College of Obstetricians and Gynecologists. Retrieved June 1, 2023, from https://www.acog.org/womens-health/infographics/changes-during-pregnancy
4. McLaughlin, J. E. (2022). Healthy Living: Menstrual Cycle. Merck Manual. Retrieved June 1, 2023, from https://www.merckmanuals.com/home/women-s-health-issues/biology-of-the-female-reproductive-system/menstrual-cycle
5. American College of Obstetricians and Gynecologists. (2019). FAQs: Fertility Awareness-Based Methods of Family Planning. American College of Obstetricians and Gynecologists. Retrieved June 1, 2023, from https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning
6. American College of Obstetricians and Gynecologists. (2021). FAQs: Bleeding During Pregnancy. American College of Obstetricians and Gynecologists. Retrieved June 1, 2023, from https://www.acog.org/womens-health/faqs/bleeding-during-pregnancy
7. National Library of Medicine. (n.d.). Medline Plus: Pregnancy Test. National Library of Medicine. Retrieved June 1, 2023, from https://medlineplus.gov/lab-tests/pregnancy-test/
8. U.S. Food & Drug Administration. (2019). Home Use Tests: Pregnancy. U.S. Food & Drug Administration. Retrieved June 1, 2023, from https://www.fda.gov/medical-devices/home-use-tests/pregnancy
9. American College of Obstetricians and Gynecologists’ Committee on Obstetric Practice. (2017). Committee Opinion No 700: Methods for Estimating the Due Date. Obstetrics and Gynecology. Retrieved June 1, 2023, from https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date
10. Rehman, S. & Ahmed, D. (2022). Embryology, Kidney, Bladder, and Ureter. StatPearls. Retrieved June 1, 2023, from https://www.ncbi.nlm.nih.gov/books/NBK547747/
11. American College of Obstetricians and Gynecologists. (2021). FAQs: Nutrition During Pregnancy. American College of Obstetricians and Gynecologists. Retrieved June 1, 2023, from https://www.acog.org/womens-health/faqs/nutrition-during-pregnancy
12. Singh, R., & Munakomi, S. (2022). Embryology, Neural Tube. StatPearls. Retrieved June 1, 2023, from https://www.ncbi.nlm.nih.gov/books/NBK542285/