The first trimester of pregnancy lasts 14 weeks, starting from the first day of your last menstrual period through 13 weeks and 6 days. That’s roughly three and a half months. What surprises many people is that you’re already considered about four weeks pregnant by the time you miss a period and get a positive test, meaning you’ve already used up nearly a third of the trimester before you even know you’re pregnant.
Why Pregnancy Starts Before Conception
Pregnancy is dated from the first day of your last menstrual period, not the day you actually conceived. This means the first two weeks of your “pregnancy” happen before an egg is even fertilized. Conception typically occurs around week two or three, but the clock has already started.
This dating system exists because most people can identify when their last period started far more reliably than when conception occurred. It’s the universal standard used by doctors, pregnancy apps, and ultrasound measurements. So when someone says they’re six weeks pregnant, the embryo itself is closer to four weeks old. The full pregnancy lasts about 40 weeks from that last period date, divided into three trimesters of roughly 13 to 14 weeks each.
What Happens During These 14 Weeks
The first trimester is when nearly all major organ development takes place. By week five or six, cells that will form the heart begin to cluster and pulse. Arm and leg buds appear around the same time. Structures that will become the brain, spinal cord, eyes, and mouth start taking shape in these early weeks.
By the end of week eight, most of the embryo’s organs and systems have formed in a basic way. This is also the point where the terminology shifts: after week eight, what was called an embryo is now called a fetus. The remaining weeks of the trimester are spent refining those structures. By the time you reach week 14, the fetus is roughly the size of a lemon, and the most chemically sensitive period of development is winding down.
Symptoms Peak Mid-Trimester
Nausea, often called morning sickness, tends to peak during the window when organ development is most vulnerable to disruption, roughly from the first month through about week 16. For many people, the worst of it hits between weeks six and ten, driven largely by a pregnancy hormone called hCG that climbs steeply and reaches its highest levels toward the end of the first trimester. As hCG levels plateau and then gradually decrease through the second trimester, nausea often eases.
Fatigue is the other hallmark of these early weeks. Your body is building an entirely new organ (the placenta) while dramatically increasing blood volume. Most people feel a noticeable energy dip that lifts somewhere around weeks 12 to 14. Some people, though, experience nausea that stretches well beyond the first trimester and in rare cases lasts the entire pregnancy.
Miscarriage Risk Drops Significantly
Most miscarriages occur during the first trimester, which is one reason many people wait to share pregnancy news until around week 12 or 13. The risk drops sharply once a heartbeat is detected on ultrasound. One study found that by week eight, after a heartbeat is confirmed, the miscarriage risk falls to about 1.5% and continues to decrease from there. By the time you cross into the second trimester, the statistical risk is quite low.
Key Appointments and Screening
Your first major ultrasound, called a dating scan, typically happens between weeks 10 and 14. This appointment confirms how far along you are, checks for a heartbeat, and establishes your estimated due date. If the scan falls within this window, you’ll also be offered a screening for chromosomal conditions like Down syndrome. This involves measuring a small fluid-filled space at the back of the baby’s neck (called a nuchal translucency measurement) and combining it with a blood test. If your dating scan happens after 14 weeks, this particular combined screening is no longer available, and alternative testing options are offered instead.
Nutrition in the First Trimester
Folic acid is the single most important nutrient in these early weeks because it helps the neural tube, the structure that becomes the brain and spinal cord, form correctly. The CDC recommends 400 micrograms daily for anyone who could become pregnant, ideally starting before conception. If you’ve had a previous pregnancy affected by a neural tube defect, the recommendation jumps to 4,000 micrograms daily, starting at least a month before conception and continuing through the first three months.
Because so much organ development happens before many people realize they’re pregnant, this is one of the few nutrients where the advice is to start supplementing before you need it rather than after. Most prenatal vitamins contain the recommended 400 micrograms, making them a simple way to cover this base even if you’re only thinking about pregnancy as a future possibility.