The first trimester of pregnancy lasts 13 weeks and 6 days, spanning from the first day of your last menstrual period through the end of week 13. That’s roughly three months, though the way weeks are counted can feel confusing at first.
How Pregnancy Weeks Are Counted
Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you actually conceived. Since ovulation and conception typically happen around two weeks after that period starts, you’re already considered “two weeks pregnant” at the moment of conception. This means the 40-week pregnancy timeline, based on a standard 28-day cycle, includes about two weeks before a fertilized egg even exists.
This is worth understanding because it explains a common source of confusion. If you conceived three weeks ago, you’re roughly five weeks pregnant by medical dating. Your doctor, your ultrasound reports, and pregnancy apps all use this LMP-based system.
What Happens During the First Trimester
The first trimester is when the most rapid and critical development takes place. In the earliest days after conception, a single fertilized cell divides into a cluster of cells, travels through the fallopian tube, and implants in the uterine lining. For the first eight weeks after fertilization (around week 10 by LMP dating), the developing baby is called an embryo. From nine weeks after fertilization until birth, it’s called a fetus.
During these 13 weeks, all major organs begin forming. The heart, brain, spinal cord, lungs, and digestive system take shape. By the end of the first trimester, the fetus measures roughly 67 millimeters from head to rump, about the length of a lime. At six weeks, that measurement is only about 5 millimeters, smaller than a pea. Growth is exponential: by week 10, the measurement reaches around 32 millimeters, and it nearly doubles again over the next three weeks.
Common Symptoms Week by Week
Most people start noticing symptoms around week 6, with nausea (often called morning sickness) being the most well-known. It peaks between weeks 8 and 10 for most women, then gradually improves and often fades near week 13. Some people experience lingering nausea into the early second trimester.
Fatigue tends to be intense during the first trimester, driven by a surge in hormones and the energy demands of early development. Breast tenderness, frequent urination, food aversions, and heightened sense of smell are also common. These symptoms vary widely from person to person and even from one pregnancy to the next. Not everyone gets morning sickness, and having mild or no symptoms doesn’t indicate a problem.
Miscarriage Risk Drops Significantly
One reason the first trimester carries so much emotional weight is that miscarriage risk is highest during these weeks. However, the risk drops quickly as the weeks progress. Research involving women with a history of recurrent miscarriage found that seeing a heartbeat at 6 weeks corresponded to a 78% chance of the pregnancy continuing. By 8 weeks with a confirmed heartbeat, that figure rose to 98%, and by 10 weeks it reached 99.4%. Once a pregnancy passes 6 or 7 weeks with a heartbeat detected, the overall miscarriage risk falls to around 10%.
These numbers explain why many people wait until the end of the first trimester to share pregnancy news. By week 13, the statistical risk has dropped dramatically.
Screening Tests and Prenatal Visits
Most people have their first prenatal appointment between weeks 8 and 10. This visit typically includes blood work, a medical history review, and sometimes an early ultrasound to confirm the pregnancy’s location and check for a heartbeat.
Between weeks 11 and 13, a first-trimester screening is offered to check for chromosomal conditions and certain heart defects. This involves a blood test measuring two protein levels and an ultrasound that looks for extra fluid behind the baby’s neck. These results, combined with your age, give a risk estimate rather than a diagnosis. If the screening flags a higher risk, further testing is available.
Nutrition in the First Trimester
Folic acid is the single most important supplement in early pregnancy. The recommendation is 400 micrograms daily, ideally starting before conception, because the neural tube (which becomes the brain and spinal cord) forms in the very early weeks, often before many people know they’re pregnant. If you’ve had a previous pregnancy affected by a neural tube defect, the recommended dose increases to 4,000 micrograms daily, starting at least one month before conception and continuing through the first three months.
If nausea makes eating difficult, focus on small, frequent meals and whatever foods you can keep down. Prenatal vitamins are especially valuable during weeks when your diet is limited, though taking them on an empty stomach can worsen nausea. Taking them with food or before bed is a common workaround.
The Transition to the Second Trimester
Week 14 marks the start of the second trimester. For many people, this shift comes with noticeable relief: nausea fades, energy returns, and the pregnancy starts to feel more physically real. The major organs have formed, and the remaining months focus on growth and maturation rather than building new structures from scratch. By the time you cross that 13-week line, the highest-risk developmental window has passed and the pregnancy enters a more stable phase.