The first trimester lasts 13 weeks and 6 days, starting from the first day of your last menstrual period and ending at the close of week 13. That means the second trimester begins at week 14. This timing surprises many people because you aren’t actually pregnant for the first two weeks of your “pregnancy,” since the count starts before ovulation and conception even occur.
How Pregnancy Weeks Are Counted
Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you conceived. Because ovulation typically happens around day 14 of your cycle, you’re already considered “2 weeks pregnant” at the moment of conception. This system exists because most people can pinpoint when their period started but not the exact day an egg was fertilized.
Your due date and trimester milestones are based on this LMP dating, but an early ultrasound can shift things. In a large study, 40% of women who received a first-trimester ultrasound had their due date adjusted because it differed from their LMP date by more than 5 days. Before 14 weeks, an ultrasound measuring the baby from head to rump is accurate to within 5 to 7 days. If the ultrasound and LMP disagree by more than 7 days, providers will typically go with the ultrasound date.
What Happens During the First 13 Weeks
More development occurs in the first trimester than at any other point in pregnancy. By week 4, the fertilized egg has implanted into the uterine lining, the amniotic sac has formed around it, and the earliest version of the placenta is taking shape. At this point the embryo is smaller than a grain of rice.
By week 8, all of the major organs and body systems are actively developing. The hands and feet have a webbed appearance, eyes become visible, and ears begin to form. The umbilical cord is fully functional, transporting oxygen and blood to the embryo.
By week 12, every organ, limb, bone, and muscle is present. The circulatory, digestive, and urinary systems are all working. The liver is producing bile, and the fetus is already swallowing and passing amniotic fluid. From this point forward, the baby’s job is to grow and mature, not to build new structures from scratch. Week 13 marks the final week of the first trimester, when the skeleton begins hardening, particularly in the skull and the long bones of the arms and legs.
Why You Feel Worst in the First Trimester
The hormone that pregnancy tests detect, hCG, rises rapidly after implantation and peaks between weeks 8 and 11. This spike is closely linked to nausea, fatigue, breast tenderness, and food aversions. Many people describe weeks 6 through 10 as the roughest stretch of the entire pregnancy. Once hCG levels plateau and begin to decline toward the end of the first trimester, these symptoms often ease considerably.
Weight gain during this period is minimal. If you start pregnancy at a healthy weight, the expected gain is only about 1 to 4 pounds across the entire first trimester. Some people gain nothing or even lose a pound or two because of nausea. That’s generally not a concern as long as you’re able to stay hydrated and eat when you can.
Miscarriage Risk Drops Significantly
The first trimester carries the highest risk of miscarriage, which is why many people wait until it’s over to share the news. The risk drops sharply once a heartbeat is detected, usually around week 6 or 7. By week 8, the risk falls to roughly 1.5% and continues declining through the rest of the trimester. Reaching week 13 without complications is a major milestone, and the chance of pregnancy loss after that point is very low.
Key Screenings and Nutrition
The first trimester includes an important screening window between weeks 11 and 13. This involves a blood test and a specialized ultrasound that measures a pocket of fluid at the back of the baby’s neck. Together, these help assess the likelihood of certain genetic conditions. Non-invasive prenatal testing (NIPT), a blood draw that screens fetal DNA, is also available during this window.
Folic acid is the single most important supplement in the first trimester. The standard recommendation is 400 micrograms daily, ideally starting before conception and continuing through the first 12 weeks. This nutrient plays a critical role in the development of the brain and spinal cord, and most of that development is complete by the end of the first trimester. People with certain risk factors may be advised to take a higher dose of 5 milligrams.
When the Second Trimester Starts
The second trimester begins at 14 weeks 0 days. You may see some sources say it starts at week 13, which creates confusion. ACOG defines the first trimester as lasting through 13 weeks and 6 days, placing the transition firmly at the start of week 14. In practical terms, the difference is a matter of days, but if your provider schedules something “at the start of the second trimester,” they typically mean week 14.