The first trimester of pregnancy lasts 13 weeks and 6 days, covering the period from the first day of your last menstrual period through the end of week 13. That works out to just over three months. The count begins not from conception but from the start of your most recent period, which means you’re technically “pregnant” for about two weeks before an egg is even fertilized.
How Pregnancy Weeks Are Counted
Pregnancy dating follows a 280-day convention measured from the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation happening around day 14. Since most people don’t conceive until about two weeks into that cycle, the embryo is actually about two weeks younger than the gestational age your provider uses. So when you’re told you’re “6 weeks pregnant,” the embryo has been developing for roughly four weeks.
If your cycles are irregular or you’re unsure when your last period started, an early ultrasound can pin down gestational age more precisely. Ultrasound measurements taken before 14 weeks are the most accurate method for confirming how far along you are. If the ultrasound date and your period-based date differ by more than five to seven days (depending on how early the scan is done), your provider will typically adjust your due date to match the ultrasound.
What Happens During the First Trimester
Development moves remarkably fast in these 13 weeks. For the first four weeks, the fertilized egg implants in the uterine lining and the earliest structures begin forming. By week 5, a primitive heart and circulatory system are taking shape. At week 6, the neural tube along the baby’s back is closing (this eventually becomes the brain and spinal cord), and small buds appear that will become arms. By week 7, leg buds emerge.
By the end of the first trimester, all major organ systems have started developing, fingers and toes are distinct, and the embryo, now called a fetus, is roughly 2 to 3 inches long. The shift from “embryo” to “fetus” happens around week 10, marking the transition from organ formation to growth and maturation.
Symptoms and When They Peak
Most pregnancy symptoms don’t kick in until four to six weeks after conception, which corresponds to roughly weeks 6 through 8 of gestational age. Breast tenderness is often the first sign, sometimes starting as early as two weeks after conception. Nausea, commonly called morning sickness, typically shows up between weeks 6 and 8 and tends to peak around weeks 9 through 10 before gradually easing for many people as the first trimester ends. Fatigue is one of the most universal early symptoms and is most intense during the first trimester.
These symptoms are driven largely by rapidly rising hormone levels. Progesterone, sometimes called the “hormone of pregnancy,” surges after ovulation from less than 1 milligram per day to 20 to 30 milligrams per day once the corpus luteum (a temporary structure in the ovary) takes over production. Between weeks 7 and 10, the placenta gradually assumes responsibility for making progesterone, a transition that can coincide with the worst stretch of nausea and fatigue for many people.
Miscarriage Risk Drops Sharply
The first trimester carries the highest risk of pregnancy loss, which is one reason many people wait until week 12 or 13 to share the news. But the risk isn’t evenly distributed across those 13 weeks. Once a heartbeat is visible on ultrasound around 6 to 7 weeks, the chance of miscarriage drops to roughly 10%. By 8 weeks with a confirmed heartbeat, the probability of the pregnancy continuing rises to about 98%. At 10 weeks, that figure reaches 99.4%.
Reliable week-by-week miscarriage statistics before a heartbeat is detected are limited, but the overall pattern is clear: risk falls rapidly as the weeks progress, and by the time you cross into the second trimester, the likelihood of loss is very low.
Screening Tests in the First Trimester
Between weeks 11 and 13, you’ll be offered first trimester screening. This involves a blood test measuring two proteins and an ultrasound that checks for extra fluid behind the baby’s neck. Together, these results estimate the likelihood of certain chromosomal conditions. The screening is optional, and it gives a risk estimate rather than a definitive diagnosis. If results suggest elevated risk, further diagnostic testing can provide a clearer answer.
Your first prenatal visit usually happens between weeks 8 and 10, and it’s when your provider will confirm gestational age, order initial blood work, and establish a care timeline for the rest of the pregnancy.
Nutrition in the First Trimester
Folic acid is the single most important supplement in early pregnancy, ideally started before conception. The CDC recommends 400 micrograms daily for anyone who could become pregnant. Folic acid supports neural tube closure, which happens around week 6, so adequate levels in the weeks before and after conception are critical. By the time many people realize they’re pregnant, this window is already closing, which is why preconception supplementation matters.
Eating well during the first trimester can feel like a contradiction when nausea makes everything unappealing. Small, frequent meals tend to be easier to tolerate than large ones. You don’t need extra calories in the first trimester. The priority is keeping food down and staying hydrated rather than hitting a perfect dietary balance every day.