The first trimester of pregnancy lasts from the first day of your last menstrual period through 13 weeks and 6 days, making it just under 14 full weeks. That’s roughly three and a half months, though the exact calendar length varies slightly depending on when your period started.
How the Start Date Is Calculated
Pregnancy isn’t counted from conception. Instead, doctors count from the first day of your last menstrual period (LMP), even though you likely didn’t conceive until about two weeks later when ovulation occurred. This means that during “week one” and “week two” of pregnancy, you weren’t actually pregnant yet. The entire pregnancy spans about 40 weeks from that LMP date, divided into three trimesters.
This counting method can feel counterintuitive, but it’s the universal standard because most people can pinpoint their last period more reliably than the exact day of conception. When your doctor says you’re “8 weeks pregnant,” they mean 8 weeks since your last period started, which is closer to 6 weeks since the embryo began developing.
Why You’ll Hear Both “12 Weeks” and “13 Weeks”
There’s genuine confusion around whether the first trimester ends at 12 or 13 weeks. The American College of Obstetricians and Gynecologists defines the first trimester as lasting through 13 weeks and 6 days. In practice, many doctors and pregnancy apps round this to “about 12 weeks” or refer to the end of week 12 as a major milestone, partly because key screening tests and risk thresholds cluster around that point. Both are used in conversation, but the official medical boundary is 13 weeks and 6 days.
What Happens During These 14 Weeks
The first trimester is when the most dramatic development occurs. By week 5, the heart and circulatory system begin forming. By week 6, the neural tube along the baby’s back is closing, which will become the brain and spinal cord. All major organs start taking shape during this window, a process called organogenesis. By the end of the trimester, the fetus has gone from a cluster of cells to a recognizable form with arms, legs, fingers, and toes.
In size terms, the embryo measures about 16 millimeters (roughly the size of a kidney bean) at 8 weeks. By 12 weeks, it reaches about 54 millimeters, or just over two inches, roughly the length of a lime.
Why You Feel Worst in the First Trimester
The pregnancy hormone hCG rises rapidly in the early weeks and peaks between weeks 8 and 12, reaching levels of 32,000 to 210,000 units per liter. This hormonal surge is a major driver of nausea, fatigue, and food aversions. For most people, these symptoms are at their worst right around that 8 to 10 week mark and begin improving as hCG levels taper off toward the end of the first trimester and into the second.
Breast tenderness, frequent urination, and mood changes are also common during this stretch. These symptoms can feel relentless, but they’re directly tied to the hormonal environment that supports early development. The relief many people describe in the second trimester isn’t imagined. It corresponds to a real shift in hormone levels.
Miscarriage Risk Drops Sharply
One of the reasons the 12-week mark carries so much emotional weight is that miscarriage risk drops dramatically by that point. About 80% of miscarriages happen in the first trimester. But the risk isn’t evenly distributed across those weeks. Once a heartbeat is visible on ultrasound around 6 to 7 weeks, the risk of miscarriage falls to about 10%. A study of more than 300 women with a history of recurrent miscarriage found that seeing a heartbeat at 8 weeks meant a 98% chance the pregnancy would continue. By 10 weeks, that figure rose to 99.4%.
This is why many people choose to share pregnancy news after the first trimester ends. The statistical risk has dropped to its lowest point, and the pregnancy has cleared the most vulnerable developmental window.
First Trimester Screening Tests
Several important prenatal tests happen in the first trimester. The most time-sensitive is the first trimester screening, which takes place between weeks 11 and 13. This involves an ultrasound that measures fluid at the back of the baby’s neck (called a nuchal translucency scan) combined with a blood test. Together, these screen for chromosomal conditions like Down syndrome.
This test has a narrow window. If you miss the 11 to 13 week range, the measurement becomes unreliable and you’ll need a different screening approach in the second trimester. Your first prenatal appointment, which typically happens between weeks 8 and 10, is when your provider will schedule this and any other early tests, including blood work to check your blood type, iron levels, and immunity to certain infections.
What Changes at 14 Weeks
Crossing into the second trimester at 14 weeks doesn’t trigger a sudden shift, but the transition is meaningful. The placenta takes over hormone production from the ovaries, which is largely why nausea fades. The risk of miscarriage is now very low. Energy levels often rebound noticeably. The fetus has completed the critical phase of organ formation and shifts into a period of growth and refinement, gaining weight and developing more complex functions like swallowing and movement you’ll eventually be able to feel.
For most people, the second trimester feels markedly easier than the first. The 14-week line isn’t just a calendar milestone. It reflects a genuine biological shift in both your body and the developing pregnancy.