The first trimester of pregnancy lasts 13 weeks, covering week 1 through the end of week 13. That said, the way pregnancy weeks are counted can be confusing, because the clock starts before you’re actually pregnant. Understanding how those 13 weeks are measured, and what happens during them, helps make sense of everything from your due date to your symptoms.
Why the Count Starts Before Conception
Pregnancy is dated from the first day of your last menstrual period, not from the day you conceived. Conception typically happens about two weeks after that date, during ovulation. So during weeks 1 and 2 of your “pregnancy,” there’s no embryo yet. This dating method exists because most people know when their last period started but can’t pinpoint the exact day of conception.
This also means you’re usually considered about 4 weeks pregnant by the time you miss a period and get a positive test. The full pregnancy spans roughly 40 weeks from that last-period start date, and dividing those 40 weeks into thirds gives you three trimesters of about 13 weeks each. Your first trimester runs from week 1 through week 13, the second from week 14 through week 27, and the third from week 28 through delivery.
What Happens Week by Week
Even though the first trimester is only 13 weeks, the pace of development is remarkable. Here’s what’s going on at key points:
Weeks 1 and 2: Your body is preparing for ovulation. No conception has occurred yet, but these weeks are part of the official count.
Week 3: Sperm and egg meet in the fallopian tube, forming a single cell with 46 chromosomes. This cell begins dividing rapidly as it travels toward the uterus.
Week 4: The ball of dividing cells burrows into the uterine lining, a process called implantation. This is roughly when a pregnancy test can first turn positive.
Weeks 5 and 6: A primitive heart and circulatory system begin forming. The foundations of the lungs, intestines, bones, and kidneys are laid down. By week 6, the brain and spinal cord are taking shape as the neural tube along the back closes. Tiny arm buds appear, and the body curves into a C shape.
Week 7: The brain and face grow quickly. Small pits that will become nostrils are visible, and the earliest structures of the eyes start forming.
Weeks 8 through 10: Fingers and toes develop and lose their initial webbing. Eyelids and outer ears continue to take shape. By the end of week 10, most major organs have at least begun forming.
Weeks 11 through 13: Starting at week 11, the developing baby is officially called a fetus rather than an embryo. The face is broad, the eyes are widely separated, and tooth buds appear. Red blood cells start forming in the liver. By the end of week 13, the fetus is roughly 3 inches long.
Symptoms You Can Expect
The first trimester is when symptoms tend to hit hardest, driven largely by a surge in hormones. The pregnancy hormone hCG rises sharply during weeks 5 through 12, peaking somewhere in the range of 32,000 to 210,000 µ/L in the blood between weeks 8 and 12. That steep climb is behind many of the hallmark symptoms.
Morning sickness typically starts between weeks 4 and 9. Despite the name, nausea can strike at any time of day and ranges from mild queasiness to frequent vomiting. For most people, it eases as the first trimester ends, though some carry it into the second.
Breast tenderness often shows up early, sometimes within the first few weeks. Your body adjusts to the hormonal changes over time, and the soreness usually becomes less intense after several weeks. Fatigue is another dominant symptom during the first trimester, driven by rising progesterone levels. Many people describe an exhaustion that sleep doesn’t fully fix, which is normal and generally improves in the second trimester.
Other common experiences include frequent urination, food aversions or cravings, mood swings, and mild cramping. Not everyone gets every symptom, and having fewer symptoms doesn’t indicate a problem.
Miscarriage Risk During the First Trimester
Most pregnancy losses happen in the first trimester, which is why many people wait until week 12 or 13 to share the news. The risk drops significantly once a heartbeat is detected. One study of over 300 women found that seeing a heartbeat at 6 weeks meant a 78% chance of the pregnancy continuing. By 8 weeks with a confirmed heartbeat, that figure rose to 98%, and by 10 weeks it reached 99.4%.
After a heartbeat is visible at 6 or 7 weeks, the overall miscarriage risk falls to around 10%. These numbers can be reassuring if you’re anxiously counting the days through those early weeks.
Your First Prenatal Appointment
You should schedule your first prenatal visit as soon as you find out you’re pregnant. This appointment is typically the longest one you’ll have because it covers a lot of ground: a physical exam, blood work to check your blood type and screen for infections like hepatitis B and HIV, a urine test to rule out a urinary tract infection, and an early discussion about genetic screening options such as tests for Down syndrome.
Your provider will also confirm your due date, usually based on the first day of your last period. If there’s any uncertainty about your dates, an early ultrasound can pin down how far along you are. This is the visit where you’ll get a clearer picture of your timeline and what to expect over the coming months.
Why You Might See Different Week Ranges
If you’ve been reading pregnancy resources, you may have noticed that some sources say the first trimester ends at week 12 and others say week 13 or even week 14. There’s no single universal cutoff. The most widely used definition places the boundary at the end of week 13 (13 weeks, 6 days), which divides 40 weeks of pregnancy into three roughly equal parts. Some providers and organizations round differently, placing the second trimester start at week 12 or week 14. The practical difference is small, and your care won’t change based on which definition your provider uses.