The first trimester lasts 13 weeks and 6 days, starting from the first day of your last menstrual period. That works out to just under 14 weeks, or roughly three months. It ends as you enter week 14, when the second trimester begins.
Why the Count Starts Before Conception
The timeline can feel confusing because pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you actually conceived. Since ovulation typically happens around day 14 of a 28-day cycle, you’re already considered “2 weeks pregnant” at the moment of conception. This means the first trimester includes about two weeks when you weren’t technically pregnant at all.
This convention assumes a regular 28-day cycle with ovulation on day 14, which doesn’t apply to everyone. If your cycles are irregular, or if you conceived through IVF, your provider may adjust the dating based on an early ultrasound or the date of embryo transfer. Either way, the first trimester still spans the same window of development.
What Happens During These 14 Weeks
The first trimester is when all major organ systems form. In the earliest weeks after conception, a fertilized egg implants in the uterine lining and begins dividing rapidly. By around week 6, a heartbeat can often be detected on ultrasound. The developing organism is called an embryo until week 10, when it transitions to being called a fetus. By the end of week 13, the basic architecture of the brain, heart, lungs, limbs, and digestive system is in place, though none of these are fully mature yet.
This rapid construction is why the first trimester is the most sensitive period for exposure to alcohol, certain medications, and infections. It’s also why folic acid supplementation matters most during these early weeks. The recommended dose is 400 micrograms daily, ideally starting three months before conception and continuing through the first 12 weeks. If you have a higher risk of neural tube defects, your doctor may recommend a much larger dose of 5 milligrams.
Symptoms and When They Peak
Up to 70% of pregnant women experience nausea during the first trimester, commonly called morning sickness despite the fact that it can strike at any time of day. For most people, it feels worst between weeks 8 and 10, then gradually eases as you approach the second trimester. Fatigue, breast tenderness, frequent urination, and food aversions are also typical.
These symptoms are driven partly by a hormone called hCG, which rises dramatically in early pregnancy. At 4 weeks, blood levels range from 0 to 750 units per liter. By weeks 8 through 12, they can climb to between 32,000 and 210,000 units per liter. That steep rise is what makes the first trimester feel so physically intense for many women, even though the pregnancy may not be visible yet. Symptoms vary enormously from person to person, and having mild or no nausea doesn’t indicate a problem.
Miscarriage Risk by Week
The first trimester carries the highest risk of miscarriage, which is one reason many people wait until the second trimester to share the news. But the risk drops sharply with each passing week. A study of over 300 women found that once a heartbeat was visible at 6 weeks, there was a 78% chance the pregnancy would continue. By 8 weeks with a confirmed heartbeat, that figure rose to 98%. At 10 weeks, it reached 99.4%. So while the first trimester can feel uncertain, the odds improve quickly.
Prenatal Screening in the First Trimester
Several important tests happen during this window. Between weeks 11 and 13, your provider may offer a first trimester screening that combines a blood test with an ultrasound measuring fluid at the back of the baby’s neck. This screening estimates the risk of certain chromosomal conditions. Non-invasive prenatal testing, which analyzes fragments of fetal DNA in your blood, can also be done starting around week 10.
Your first prenatal visit typically happens between weeks 8 and 10 and includes bloodwork, a medical history review, and often a dating ultrasound to confirm how far along you are.
Weight Changes to Expect
Weight gain in the first trimester is minimal. Most people gain between 1 and 5 pounds during the first 12 weeks, and some gain no weight at all. If nausea is severe, you might even lose a pound or two. None of this is cause for concern. The more significant weight gain happens in the second and third trimesters, and the recommended total depends on your pre-pregnancy BMI.