The first trimester of pregnancy lasts 13 weeks and 6 days, starting from the first day of your last menstrual period (LMP) through the end of week 13. That means the second trimester begins at week 14. These 13-plus weeks are when the most dramatic development happens, transforming a single fertilized cell into a fetus with all major organs in place.
Why Pregnancy Weeks Start Before Conception
One detail that catches many people off guard: you aren’t actually pregnant during the first roughly two weeks of your “pregnancy.” Doctors date pregnancy from the first day of your last period, not from conception. A typical pregnancy lasts 280 days, or 40 weeks, counted this way. This system, known as Naegele’s Rule, assumes a 28-day menstrual cycle, meaning conception typically happens around week 2. So when your provider says you’re “6 weeks pregnant,” the embryo has really only been developing for about 4 weeks.
This dating method exists because most people know when their last period started but can’t pinpoint the exact day of conception. It’s the universal standard used by obstetricians, midwives, and pregnancy apps alike.
What Happens Week by Week
The first trimester is when organogenesis occurs, the process of building every major organ system from scratch. The pace is remarkable.
By week 5, the neural tube that will become the brain and spinal cord starts forming, and a tiny cluster of cells begins pulsing at about 110 beats per minute. This is the earliest version of a heart. By week 6, that pulse is usually detectable on a vaginal ultrasound, and small buds that will become arms and legs appear. Structures that eventually form the ears, eyes, and mouth also take shape around this time.
Week 7 brings the beginning of bone formation, as cartilage starts hardening. By week 8, all major organs and body systems are actively developing. At week 12, all organs, limbs, bones, and muscles are present. They’ll continue maturing for the rest of the pregnancy, but the basic architecture is complete. Week 13 marks the transition point where bones in the skull and long bones of the arms and legs begin to harden further.
Common First Trimester Symptoms
Most first trimester symptoms trace back to a hormone called hCG, which your body produces in rapidly increasing amounts after implantation. Levels climb steeply through the first several weeks, peaking somewhere between weeks 8 and 12. At week 4, blood levels average under 750 units per liter. By weeks 8 through 12, they can reach 32,000 to 210,000 units per liter. That surge is what drives nausea, fatigue, breast tenderness, and food aversions. For most people, these symptoms ease as hCG levels taper heading into the second trimester.
Your cardiovascular system also starts changing earlier than you might expect. Blood vessels begin to relax as early as 6 to 8 weeks, which can lower your blood pressure by 5 to 10 points below your normal baseline. Kidney blood flow increases by up to 50% by the end of the first trimester. These shifts can contribute to dizziness, more frequent urination, and feeling lightheaded when you stand up quickly.
Miscarriage Risk Drops Significantly
The first trimester carries the highest risk of pregnancy loss, which is why many people wait until around week 12 to share their news. But the risk isn’t evenly spread across those 13 weeks. It drops sharply once a heartbeat is confirmed. 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 visible heartbeat, that rose to 98%. At 10 weeks, it reached 99.4%.
By the end of week 12, the risk of miscarriage falls substantially, which is well supported across multiple studies. This is also when many providers schedule the first major screening tests.
First Trimester Screening and Appointments
Your first prenatal visit typically happens between weeks 8 and 10, though timing varies by provider. The most significant screening window falls between weeks 11 and 13. During this window, a combined screening involves a blood test and a nuchal translucency ultrasound, which measures fluid at the back of the baby’s neck. Together, these help assess the chances of certain genetic conditions.
This is also the period when many providers offer cell-free DNA screening (sometimes called NIPT), a blood test that analyzes fragments of fetal DNA circulating in your bloodstream. Both tests are optional, and your provider will walk you through what the results can and can’t tell you.
Nutrition in the First 13 Weeks
Folic acid is the single most important supplement during the first trimester because the neural tube forms so early, often before many people even know they’re pregnant. The CDC recommends 400 micrograms daily for anyone who could become pregnant. If you’ve had a previous pregnancy affected by a neural tube defect, the recommendation jumps to 4,000 micrograms daily, starting at least one month before conception and continuing through the first three months.
Nausea can make eating feel like a chore during these weeks. Small, frequent meals tend to be easier to manage than three large ones. Keeping something bland by your bedside for the first few minutes after waking can help take the edge off morning sickness before you’re fully upright.