Your first trimester lasts 13 weeks, counted from the first day of your last menstrual period through the end of week 13. That’s roughly three calendar months. But the counting method trips up a lot of people, because you’re not actually pregnant for the first two weeks of that window.
Why Pregnancy Weeks Start Before Conception
Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you actually conceived. Ovulation and conception typically happen around week 2, so by the time you miss a period and get a positive test, you’re already considered about 4 weeks pregnant. This means your first trimester includes roughly two weeks before conception even occurred.
The gap matters because it explains why the math can feel off. You may have only known about your pregnancy for 9 or 10 weeks by the time the first trimester ends, even though you’re technically 13 weeks along. Most pregnancies last about 40 weeks total from the LMP, and the first trimester covers the first third of that timeline.
What Happens to Your Baby During These 13 Weeks
The first trimester is when the most dramatic development occurs. By week 4, the fertilized egg has burrowed into the uterine lining and begun forming the earliest structures that will become the embryo and the placenta. At this stage, you may not even know you’re pregnant yet.
By week 8, things look remarkably different. Fingers have started to form, the eyes are visible, and the upper lip and nose are taking shape. The trunk and neck begin to straighten. Your baby is about half an inch long, measured from the top of the head to the base of the spine.
By week 12, fingernails are growing, the face has a recognizable profile, and the intestines have moved into the abdomen. At this point, your baby is about 2.5 inches long and weighs roughly half an ounce. That’s a jump from a barely visible cluster of cells to something that looks unmistakably human in just a few months.
Common First Trimester Symptoms
Nausea, fatigue, and breast tenderness are the hallmarks of the first trimester, driven largely by a pregnancy hormone called hCG. Levels of hCG climb rapidly and peak between weeks 8 and 12, which is exactly when nausea tends to be at its worst. Blood levels during that window can range from 32,000 to 210,000 units per liter, a massive spike compared to pre-pregnancy levels of essentially zero.
Morning sickness peaks precisely when organ development is most active, roughly from the first month through about week 16. For most people it eases as the first trimester ends, but some experience nausea well into the second trimester or even throughout the entire pregnancy. The fatigue can be equally intense. Your body is building a placenta from scratch and increasing blood volume, which takes real energy even though nothing is visibly different yet.
The transition into the second trimester often brings noticeable relief. Appetite typically increases, nausea fades, and energy levels start to recover. Your abdomen begins to expand, and you may start to look pregnant rather than just feeling it.
Miscarriage Risk Drops as Weeks Pass
Between 10% and 20% of all known pregnancies end in miscarriage, and 80% of those losses happen during the first trimester, before 13 weeks. The risk decreases with each passing week. By the time you’ve reached the end of the first trimester with a confirmed heartbeat, the odds of carrying to term are substantially in your favor.
This declining risk is one reason many people wait until the end of the first trimester to share pregnancy news. It’s a personal choice, but the statistics behind it are real.
Prenatal Care in the First Trimester
Your first prenatal visit should ideally happen before 10 weeks after your last period. During early visits, expect routine checks like blood pressure and weight, along with blood tests and possibly a pelvic exam. You’ll likely have at least one ultrasound to confirm the pregnancy’s location, check for a heartbeat, and establish a more accurate due date.
Between weeks 11 and 13, you may be offered first trimester screening. This combines a blood draw with a specialized ultrasound that measures fluid at the back of the baby’s neck to assess the chance of certain genetic conditions. A blood-based screening test can also be done during this window. Both are optional, and your provider will walk you through what each one can and can’t tell you.
Folic Acid and Early Nutrition
Folic acid is the single most important supplement in the first trimester because the baby’s brain and spinal cord form very early, often before you know you’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 recommended dose jumps to 4,000 micrograms daily, starting at least a month before conception and continuing through the first three months.
Eating well during the first trimester can be challenging when nausea limits what sounds appealing. The priority is staying hydrated and getting what you can tolerate. Calorie needs don’t actually increase much in the first trimester, so the focus is more on nutrient quality than quantity.