The first trimester of pregnancy lasts about 13 weeks, starting from the first day of your last menstrual period (LMP) and ending at 13 weeks and 6 days. That means it covers roughly three calendar months, though the way pregnancy is dated can make the timeline feel slightly confusing.
How the First Trimester Is Dated
Pregnancy isn’t counted from the day of conception. Instead, doctors start the clock on the first day of your last period, even though you likely didn’t conceive until about two weeks later when ovulation occurred. This convention assumes a regular 28-day cycle with ovulation on day 14, which doesn’t apply to everyone. If your cycles are irregular or you’re unsure about the date of your last period, an early ultrasound can pin down gestational age more precisely.
First-trimester ultrasounds measure the embryo from head to rump (called crown-rump length) and are accurate to within five to seven days. The earlier in the trimester this measurement is taken, the more accurate it is. At around 8 weeks, the embryo measures roughly 16 to 22 millimeters. By 12 weeks, it’s grown to about 54 to 66 millimeters, or just over two inches long.
What Happens During These 13 Weeks
The first trimester is when all major organ development takes place. The lungs begin forming the tubes that will eventually carry air. The inner ear starts to develop. The liver begins to form. During weeks 9 through 12, cartilage for the limbs, hands, and feet appears, though it won’t harden into bone until later. Eyelids form but stay closed, and the genitals begin to take shape. By the end of this trimester, the basic blueprint for every organ system is in place, even though the fetus is still very small.
Common Symptoms and Why They Happen
Most first-trimester symptoms trace back to a sharp rise in hormones, particularly progesterone. Rising progesterone levels are why fatigue hits so hard in early pregnancy. The same hormone slows your digestive system, which can cause constipation. It also relaxes the valve between your stomach and esophagus, letting acid creep upward and causing heartburn. Food cravings and aversions are driven by these hormonal shifts too.
Nausea (often called morning sickness, though it can strike at any hour) typically peaks between weeks 6 and 9 and eases for most people as the first trimester winds down. Not everyone experiences it, and severity varies widely.
Miscarriage Risk by Week
One reason the first trimester feels so high-stakes is that the risk of pregnancy loss is highest during these early weeks. Once a heartbeat is visible 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, it reaches 99.4%. After entering the second trimester, the risk of late miscarriage falls to 3 to 4%.
This steep decline in risk is part of why many people wait until the end of the first trimester to share pregnancy news, though that’s entirely a personal choice.
Screening and Nutrition Timing
Several important tests and health steps are tied to specific windows within these 13 weeks. First-trimester screening, which includes an ultrasound looking for extra fluid behind the baby’s neck as a marker for chromosomal differences or heart defects, is done between weeks 11 and 13.
Folic acid is the single most time-sensitive nutrient of the first trimester. The CDC recommends 400 micrograms daily for anyone who could become pregnant, ideally starting before conception. The neural tube, which becomes the brain and spinal cord, forms very early, often before many people even know they’re pregnant. If you’ve had a previous pregnancy affected by a neural tube defect, the recommended dose is much higher at 4,000 micrograms daily, starting at least a month before conception and continuing through the first three months.
When the First Trimester Ends
You officially enter the second trimester at 14 weeks. For many people, this transition brings noticeable relief: nausea fades, energy returns, and the risk of pregnancy loss drops significantly. The 13-week mark isn’t a sharp biological line, though. It’s a clinical milestone that reflects the shift from the period of major organ formation to a phase focused more on growth and maturation. Your body has already done an enormous amount of work by the time you cross it.