The first trimester of pregnancy lasts 12 weeks, counted from the first day of your last menstrual period. That means it covers week 1 through week 12, even though conception typically doesn’t happen until around week 2 or 3. This dating method can feel counterintuitive, but it’s the standard used by doctors worldwide because the last period date is easier to pinpoint than the exact moment of fertilization.
Why the Count Starts Before Conception
Pregnancy is dated from the first day of your last period, not from when you actually conceived. Since ovulation usually occurs about two weeks into your cycle, you aren’t technically pregnant during the first two weeks of “pregnancy.” By the time you miss a period and get a positive test, you’re generally already around 4 weeks along. This is why many people are surprised to learn they’re already a month into their pregnancy before they even know about it.
What Happens During Each Phase
The first trimester packs an enormous amount of development into a short window. During these 12 weeks, a single fertilized cell transforms into a recognizable human form with functioning organs.
Weeks 1 Through 4
The first two weeks involve your body preparing for ovulation and the egg being fertilized. By week 3, the fertilized egg implants in the uterine wall. By week 4, the structure that will become the placenta starts forming, and the very earliest foundations of the brain and spinal cord are laid down. This is when a pregnancy test first turns positive.
Weeks 5 Through 8
This is the most intensive period of organ formation. The heart begins beating, and tiny limb buds appear that will become arms and legs. Muscles of the eyes, nose, and mouth form. By the end of this stretch, webbed fingers and toes poke out from developing hands and feet, and the lungs start building the tubes that will eventually carry air in and out. The embryo grows from roughly the size of a sesame seed to the size of a raspberry.
Weeks 9 Through 12
The embryo is now called a fetus. Lungs begin forming tissue that will later handle oxygen exchange. Bones start to harden, and external genitalia begin to develop, though it’s still too early to determine sex on an ultrasound. By the end of week 12, all major organs and systems are in place, and the rest of pregnancy is largely about growth and maturation.
Symptoms and When They Peak
Most of the classic early pregnancy symptoms are driven by a hormone called hCG, which your body produces in rapidly increasing amounts after implantation. Levels climb steeply from around 200 units at week 5 to a peak range of 32,000 to 210,000 units between weeks 8 and 12. This hormonal surge is directly responsible for many of the symptoms that define the first trimester experience.
Morning sickness typically begins one to two months after conception and can strike at any time of day, not just mornings. For most people, it eases as hCG levels plateau and then drop heading into the second trimester. Breast tenderness often shows up in the first few weeks as one of the earliest signs, then gradually decreases as your body adjusts. Fatigue is another hallmark, often hitting hardest in the early weeks when your body is channeling enormous energy into building a placenta and supporting rapid embryonic growth.
The good news: because hCG levels decline after the first trimester, many of these symptoms fade noticeably once you cross into week 13. This is why the second trimester is often described as the most comfortable stretch of pregnancy.
Miscarriage Risk Drops Sharply by Week
One reason the first trimester carries so much emotional weight is that the risk of pregnancy loss is highest during these weeks. Roughly 10 to 20 percent of known pregnancies end in early miscarriage, with the vast majority occurring before week 12.
The numbers shift dramatically once a heartbeat is detected. A study of over 300 women showed that seeing a heartbeat at 6 weeks gave a 78 percent chance of the pregnancy continuing. By 8 weeks with a confirmed heartbeat, that number jumped to 98 percent. At 10 weeks, it reached 99.4 percent. This steep drop in risk is a big part of why many people choose to wait until the end of the first trimester to share pregnancy news.
First Trimester Screening and Appointments
Your first prenatal visit usually happens between weeks 8 and 10. This appointment establishes your due date (typically confirmed by ultrasound), runs baseline blood work, and reviews your health history.
The main screening window falls between weeks 11 and 13. This involves two components: a blood test measuring two proteins in your blood, and an ultrasound that checks for extra fluid behind the baby’s neck. Together, these help assess the likelihood of certain chromosomal conditions and heart defects. The results don’t give a diagnosis on their own. They flag whether further testing, like a more detailed blood draw that analyzes fetal DNA, would be recommended.
If you’re unsure about timing, the key dates to keep in mind are simple. Get your first appointment by week 10, and schedule your first trimester screening between weeks 11 and 13. Missing that screening window means waiting for the second trimester tests, which run between weeks 15 and 20 and measure different markers.
When the First Trimester Ends
The first trimester officially wraps up at the end of week 12, with some sources extending it to 13 weeks and 6 days. Either way, crossing this threshold marks a meaningful transition. The risk of pregnancy loss drops to its lowest point, the worst of the nausea and fatigue often begins to lift, and the intensive phase of organ formation is complete. From here, the second trimester runs from week 13 (or 14) through week 27, and the third trimester carries you from week 28 to delivery around week 40.