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 you’re in your first trimester for roughly the first three months of pregnancy, though the math isn’t a perfect calendar-month conversion since months vary in length.
Why Pregnancy Starts Before Conception
One detail that surprises many people: your pregnancy technically begins before you’ve actually conceived. Doctors count from the first day of your last menstrual period (called gestational age), not from the day sperm meets egg. Since ovulation typically happens around day 14 of a 28-day cycle, there’s about a two-week gap between the “start” of pregnancy and actual fertilization. So when you get a positive test and your doctor says you’re four weeks pregnant, the embryo itself is closer to two weeks old.
This dating method exists because most people can identify when their last period started, while pinpointing the exact day of conception is much harder. Every week count you’ll see on pregnancy apps, in medical guidelines, and on ultrasound reports uses this gestational age system.
What Happens Week by Week
The first trimester is when every major organ system begins to form, which is why it’s often called the most critical developmental window of pregnancy. Growth during these weeks is remarkably fast.
At week 4, the embryo is about the size of a poppy seed. By week 6, it’s the size of a lentil (roughly a quarter inch long), and a heartbeat can often be detected on ultrasound. At week 7, it doubles to about half an inch, comparable to a blueberry. By week 9, your baby is grape-sized and nearly an inch long. At the end of week 12, it’s about the size of a lime, just over two inches long and weighing around two ounces. In the span of roughly eight weeks of active development, the embryo goes from a cluster of cells to a recognizable human form with arms, legs, fingers, and toes.
Common First Trimester Symptoms
Most first trimester symptoms trace back to one hormone: human chorionic gonadotropin, or hCG. Your body starts producing it right after implantation, and levels climb steeply through the early weeks. They peak between weeks 8 and 12, reaching concentrations of 32,000 to 210,000 units per liter. That peak is why nausea (often called morning sickness, though it can hit any time of day) tends to be worst during those middle weeks of the first trimester and then gradually eases as you approach the second trimester.
Fatigue is the other hallmark symptom. Your body is building a placenta, increasing blood volume, and running a massive construction project at the cellular level. Breast tenderness, frequent urination, and food aversions are also common. Not everyone experiences all of these, and severity varies widely from one pregnancy to another.
Miscarriage Risk Drops Significantly
The first trimester carries the highest risk of pregnancy loss, which is one reason many people wait until it’s over to share their news. At six weeks, the estimated risk of miscarriage is about 9.4%. By eight weeks, that drops sharply to around 1.5%, and it continues to fall with each passing week. Once you’ve reached the end of the first trimester with a confirmed heartbeat and normal growth, the overall risk of loss drops to well under 1%.
Prenatal Tests in the First Trimester
Your first prenatal appointment usually happens somewhere between weeks 8 and 10. It typically includes blood work, a medical history review, and sometimes an early ultrasound to confirm the pregnancy’s location and estimate your due date.
Between weeks 11 and 13, you’ll be offered first trimester screening. This combines a blood test with an ultrasound that measures the fluid at the back of the baby’s neck (called a nuchal translucency scan). Together, these help assess the likelihood of certain chromosomal conditions and heart defects. The screening doesn’t give a diagnosis. It gives a risk estimate, and if the results suggest higher risk, your doctor will discuss follow-up diagnostic testing.
Nutrition That Matters Most Early On
Folic acid is the single most important supplement during the first trimester, and ideally you’d start taking it at least a month before conception. The recommended dose is 400 to 800 micrograms daily. It plays a direct role in preventing neural tube defects, which are birth defects of the brain and spine that develop very early, often before you even know you’re pregnant. Most prenatal vitamins contain the right amount, but it’s worth checking the label to confirm.
Beyond folic acid, staying hydrated and eating what you can tolerate matters more than following a perfect diet, especially if nausea is making meals difficult. The baby’s nutritional demands are still small during the first trimester, so the priority is consistency over perfection.