The first trimester lasts 13 weeks and 6 days, starting from the first day of your last menstrual period. That means it covers roughly the first three months of pregnancy, wrapping up at the end of week 13. What surprises many people is that the clock starts ticking about two weeks before conception actually happens.
Why Pregnancy Starts Before Conception
Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day sperm meets egg. This convention assumes a 28-day cycle with ovulation around day 14, so by the time fertilization occurs, you’re already considered about two weeks pregnant. It’s a quirk of medical dating that can make the math feel off, but it’s the standard used worldwide.
This is why a “positive test at 4 weeks” doesn’t mean you conceived a month ago. You likely conceived roughly two weeks prior, around the time of ovulation. An early ultrasound, ideally before 13 weeks and 6 days, is the most accurate way to confirm or adjust your due date, especially if your cycles are irregular or you aren’t sure when your last period started.
What Happens During These 13 Weeks
The first trimester is when all major organ systems take shape. Development moves fast, and most of the foundational work happens before many people even realize they’re pregnant.
By week 5, a primitive heart and circulatory system begin forming. By week 6, the neural tube along the baby’s back is closing (this structure becomes the brain and spinal cord), and small buds that will become arms appear. Lower limb buds follow in week 7. By the end of week 12, all major organs have at least a basic structure in place, though they’ll continue maturing for months.
Because this period involves so much critical development, it’s also when the pregnancy is most vulnerable to disruption. This is why folic acid, which helps the neural tube close properly, is recommended before conception and through the first three months. The CDC advises 400 micrograms daily for all women who could become pregnant.
How Miscarriage Risk Changes Week by Week
Most pregnancy losses happen in the first trimester, but the risk drops sharply as the weeks progress. Once a heartbeat is visible on ultrasound at 6 weeks, the chance of miscarriage falls to around 10%. By 8 weeks with a confirmed heartbeat, the probability of the pregnancy continuing rises to about 98%. At 10 weeks, it reaches 99.4%.
These numbers come from studies of women with a history of recurrent miscarriage, so for someone without that history, the odds are generally even more favorable. Still, the steep decline in risk from week 6 to week 10 explains why many people choose to wait until the end of the first trimester before sharing pregnancy news.
What the First Trimester Feels Like
The hormonal shift in the first trimester is dramatic. Levels of hCG, the hormone that pregnancy tests detect, rise from nearly zero at week 4 to a peak range of 32,000 to 210,000 units per liter between weeks 8 and 12. That surge is largely responsible for the signature symptoms: nausea, fatigue, breast tenderness, and heightened sense of smell.
Nausea (often called morning sickness, though it can hit at any hour) typically peaks between weeks 8 and 10. For most people it begins improving around week 13, right at the end of the first trimester. Some women experience lingering nausea into the early second trimester, but the worst of it is usually concentrated in that 8-to-10 week window. Fatigue can be just as intense during this stretch, partly because your body is building an entirely new blood supply and placenta while hCG levels climb.
First Trimester Screening and Prenatal Visits
Your first prenatal visit usually happens between weeks 8 and 10. It typically includes blood work, a medical history review, and sometimes an early ultrasound to confirm dating and check for a heartbeat.
The main screening window comes between weeks 11 and 13. This involves a blood test measuring two proteins (hCG and PAPP-A) combined with an ultrasound that checks for extra fluid behind the baby’s neck. Together, these look for signs of certain chromosomal conditions and heart defects. The results don’t give a diagnosis but rather a risk estimate, which helps you decide whether further testing makes sense.
Because this screening has a narrow window, knowing exactly where you are in the first trimester matters. If your dating is off by even a week or two, it can affect the accuracy of results. That’s another reason early ultrasound dating is so valuable.
When the First Trimester Ends
According to ACOG, the first trimester officially ends at 13 weeks and 6 days. You’ll see some sources round this to “12 weeks” or “the end of month 3,” which is close enough for casual conversation but not precisely right. The second trimester begins at week 14 and runs through week 27.
For most people, crossing into the second trimester brings real relief. Nausea fades, energy returns, and the risk of pregnancy loss drops to its lowest point. The first trimester is the shortest stretch of pregnancy in terms of weeks, but it often feels like the longest because of how much is happening, both inside your body and in the anxious mental math of early pregnancy.