The first trimester of pregnancy lasts 13 weeks and 6 days, starting from the first day of your last menstrual period (LMP) and ending at the close of week 13. That works out to just over three calendar months. What surprises many people is that the clock starts ticking before conception actually happens, which means you’re already considered about four weeks pregnant by the time you miss a period and get a positive test.
Why Pregnancy Starts Before Conception
Gestational age is based on the date of your last period, not the date of conception. Ovulation and fertilization typically happen around week 2 or 3, so the first two weeks of “pregnancy” are really the tail end of your previous cycle and the days leading up to ovulation. This dating method exists because most people can recall when their last period started but can’t pinpoint the exact day of conception.
This also explains why so many people don’t realize they’re pregnant until they’re already four or five weeks along. By that point, the embryo has already implanted in the uterine lining and the placenta is beginning to form.
What Happens Week by Week
The first trimester is when all the major organ systems take shape. ACOG describes it as “the time when fertilization and major organ development occurs.” For the first eight weeks after fertilization, the developing baby is called an embryo. From nine weeks after fertilization onward, it’s called a fetus.
Here’s a condensed look at key milestones:
- Weeks 3 to 4: The fertilized egg travels down the fallopian tube, divides rapidly, and burrows into the uterine wall. The inner cluster of cells will become the embryo, and the outer layer will form part of the placenta.
- Week 5: Three distinct cell layers form. These will eventually become skin, the nervous system, the heart, bones, kidneys, lungs, and intestines.
- Week 6: The neural tube along the back closes, forming the foundation of the brain and spinal cord. The heart and other organs begin taking shape, and tiny buds appear where the arms will grow.
- Weeks 7 to 8: The face starts forming. Nostrils, the beginnings of retinas, and early ear structures become visible. Fingers start to emerge. By the end of week 8, the embryo is roughly half an inch long.
- Weeks 9 to 13: Toes appear, elbows bend, and eyelids form. Organs continue to develop and the embryo officially becomes a fetus. By the end of the trimester, most major structures are in place.
Symptoms and Why They Peak
The hormone that pregnancy tests detect, hCG, rises rapidly after implantation and reaches its highest levels toward the end of the first trimester. This hormone surge is a major driver of classic early pregnancy symptoms: nausea, fatigue, breast tenderness, and food aversions. For many people, these symptoms are at their worst between weeks 8 and 12, then gradually ease as hCG levels taper off in the second trimester.
Your body also begins making more blood almost immediately. Blood volume starts increasing within the first few weeks of gestation and will eventually rise by roughly 45% above pre-pregnancy levels. Your heart rate climbs too, gradually increasing by 10 to 20 beats per minute over the course of the full pregnancy. These cardiovascular shifts can contribute to the lightheadedness and fatigue that are common in early pregnancy.
Miscarriage Risk Drops Quickly
One reason the first trimester feels so nerve-wracking is that most pregnancy losses happen during this window. But the risk drops sharply as the weeks pass. Once a heartbeat is detected around 6 to 7 weeks, the chance of miscarriage falls to around 10%. By 8 weeks with a confirmed heartbeat, the probability of the pregnancy continuing rises to about 98%. At 10 weeks, that number reaches 99.4%.
This is why many people choose to share pregnancy news after the first trimester ends. By week 13, the statistical risk of loss has dropped dramatically, and the major organs have finished their initial formation.
Screening and Prenatal Care
Your first prenatal appointment usually falls between weeks 8 and 10 and includes bloodwork, a medical history review, and often an early ultrasound to confirm the pregnancy’s location and estimate gestational age.
The main screening window in the first trimester comes at 11 to 13 weeks. This involves an ultrasound that measures fluid behind the baby’s neck, a marker used to assess the likelihood of certain chromosomal conditions. A blood test is typically done at the same time. Cell-free DNA screening, which analyzes fragments of the baby’s DNA circulating in your blood, can also be done starting around week 10.
Nutrition in the First Trimester
Folic acid is the single most important nutrient in the first trimester because the neural tube, which becomes the brain and spinal cord, closes around week 6. The CDC recommends 400 micrograms of folic acid daily for anyone who could become pregnant, ideally starting at least a month before conception. If you’ve had a previous pregnancy affected by a neural tube defect, the recommended dose jumps to 4,000 micrograms daily, starting a month before pregnancy and continuing through the first three months.
Since many people don’t know they’re pregnant until weeks 4 or 5, taking folic acid before conception is the best way to ensure adequate levels during the critical window when the neural tube is forming. Most prenatal vitamins contain the standard 400 to 800 microgram dose.