The first trimester lasts about 13 weeks, running from the first day of your last menstrual period through 13 weeks and 6 days. That means it covers roughly the first three months of pregnancy, though the math can feel confusing because the count starts before conception actually happens.
Why the Count Starts Before Conception
Pregnancy weeks are measured from the first day of your last menstrual period (LMP), not from the day you conceived. Fertilization typically happens around week 2 or 3 of that timeline, which means you’re already considered “4 weeks pregnant” by the time you miss a period and get a positive test. This dating method exists because most people can pinpoint their last period more reliably than the exact day of conception.
So while the first trimester spans about 14 calendar weeks on paper, the embryo itself is only developing for roughly 11 to 12 of those weeks. The full pregnancy lasts about 40 weeks from the LMP, divided into three trimesters: weeks 1 through 13, weeks 14 through 27, and weeks 28 through 40.
What Happens Week by Week
The first trimester is when all major organ development takes place. For the first eight weeks after fertilization, the developing baby is called an embryo. From nine weeks after fertilization until birth, it’s called a fetus. Here’s what that progression looks like in practice.
During weeks 1 through 4, the fertilized egg implants into the uterine wall and the very earliest cell division begins. Most people don’t know they’re pregnant yet. By weeks 5 and 6, a heartbeat can often be detected on ultrasound, and the embryo measures just 5 to 9 millimeters, roughly the size of a small seed.
Weeks 7 and 8 bring rapid growth. The embryo roughly doubles in size, reaching about 16 to 22 millimeters. Limb buds appear, and the brain, spinal cord, and digestive tract are forming. By weeks 9 through 12, cartilage for the limbs, hands, and feet starts to develop (though it won’t harden into bone for several more weeks). Eyelids form but stay closed. The lungs begin building the airways that will carry air after birth, the inner ear starts to develop, the liver takes shape, and genitals begin to form.
By the end of week 13, the embryo-turned-fetus measures about 67 to 80 millimeters from head to rump, a little over 3 inches. That’s a dramatic jump from the 5-millimeter speck at week 6.
Common Symptoms and Why They Peak
The hormone driving most first-trimester symptoms is hCG (human chorionic gonadotropin), which your body produces in rapidly increasing amounts after implantation. HCG levels peak around week 10, which is why nausea, fatigue, and breast tenderness often feel worst between weeks 8 and 12. After that peak, levels gradually decline, and many people notice their symptoms easing as they approach the second trimester.
Other common experiences during these weeks include food aversions, heightened sense of smell, frequent urination, and mood swings. Not everyone gets morning sickness, and the severity varies widely. Feeling terrible doesn’t mean something is wrong, and feeling fine doesn’t mean something is wrong either.
First Trimester Screening
Most prenatal screening happens in a narrow window toward the end of the first trimester. Between weeks 11 and 13, your provider may offer a nuchal translucency (NT) scan, an ultrasound that measures a small pocket of fluid at the back of the baby’s neck. This measurement, combined with a blood test, helps estimate the chances of certain genetic conditions like Down syndrome.
Non-invasive prenatal testing (NIPT), a blood draw that analyzes fragments of fetal DNA circulating in your bloodstream, is also offered during the first trimester. Both tests are screening tools, not diagnostic ones, meaning they estimate risk rather than confirm a diagnosis. If results come back elevated, further testing is an option.
Why the First Trimester Is a Distinct Phase
The reason the first 13 weeks are grouped together isn’t arbitrary. This is the period of organogenesis, when every major organ system is being built from scratch. That’s what makes it different from the second and third trimesters, when those organs are mainly growing and maturing rather than forming for the first time. It’s also the window when the pregnancy is most vulnerable to disruption, which is why miscarriage risk drops significantly once you cross into week 14.
By the time you reach the end of week 13, the foundation for every organ is in place, the fetus is about 3 inches long, and you’re one-third of the way through pregnancy.