Each trimester of pregnancy lasts roughly 13 to 14 weeks. A full pregnancy spans 40 weeks (280 days) from the first day of your last menstrual period, and that total is divided into three trimesters of approximately equal length.
How the Three Trimesters Break Down
The NHS defines the trimesters as follows: the first trimester runs from week 4 through week 12, the second from week 13 through week 27, and the third from week 28 through week 41. That makes the first trimester about 9 weeks long from the point most people learn they’re pregnant, the second trimester about 15 weeks, and the third around 12 to 13 weeks.
You’ll sometimes see slightly different cutoffs depending on the source. Some providers count the first trimester as weeks 1 through 12 (including the two weeks before conception actually occurs), while others start counting from week 4, when a missed period first signals pregnancy. The second and third trimester boundaries can also shift by a week depending on the system used. These small differences don’t change your care; they’re just different ways of slicing the same 40-week timeline.
Why Pregnancy Starts Before Conception
The 40-week clock starts on the first day of your last menstrual period, not the day you conceived. Because ovulation typically happens around day 14 of a 28-day cycle, about two weeks of “pregnancy” pass before an egg is even fertilized. This method, called gestational age dating, is the universal standard because most people can recall when their last period started but can’t pinpoint the exact day of conception.
Your due date is calculated using a simple formula known as Naegele’s Rule: take the first day of your last period, count back three calendar months, then add one year and seven days. So if your last period started on March 10, you’d count back to December 10 and add a year and seven days, landing on a due date of December 17. The formula assumes a regular 28-day cycle, so if your cycles are longer or shorter, your provider may adjust the estimate with an early ultrasound.
For pregnancies conceived through IVF or other assisted reproduction, the due date is calculated from the date the embryo was transferred rather than a menstrual period. A day-5 embryo, for example, puts the due date at 261 days from the transfer date.
What Happens in Each Trimester
First Trimester (Weeks 1 Through 12)
This is when the foundation of every major organ system is laid down. By week 5, the neural tube that will become the brain and spinal cord starts forming. By week 6, tiny limb buds appear. By the end of week 12, all organs, limbs, bones, and muscles are present in basic form. They’ll spend the rest of pregnancy growing and maturing, but the blueprint is complete by the end of the first trimester.
For you, the first trimester is often the hardest in terms of symptoms like nausea, fatigue, and breast tenderness, even though you may not look pregnant yet. These symptoms are driven by a rapid surge in hormones that your body gradually adjusts to.
Second Trimester (Weeks 13 Through 27)
Often called the most comfortable stretch. Nausea typically fades, energy returns, and your bump becomes visible. Meanwhile, the fetus is growing rapidly. Around week 14, skin starts to thicken and fine hair appears. By week 19, movements are strong enough that most people feel kicks and punches for the first time. By week 24, the lungs are structurally developed but not yet mature enough to function outside the uterus.
Third Trimester (Weeks 28 Through 40+)
The final stretch focuses on growth and maturation. By week 30, the fetus can regulate its own body temperature. By week 32, its skin is no longer translucent. By week 36, the fine body hair (lanugo) that covered it for months falls away, and head hair fills in. The fetus gains significant weight during these last weeks, building the fat stores it needs for life outside the womb.
For you, this trimester brings increasing physical strain: back pain, shortness of breath as the uterus pushes against your diaphragm, and frequent urination as it presses on your bladder. Most babies settle into a head-down position by week 36 in preparation for delivery.
When Pregnancy Is Considered “Full Term”
Not all births at 37 weeks or later are equal. In 2013, ACOG replaced the blanket label of “term” with more specific categories. A birth between 37 weeks and 38 weeks 6 days is considered early term. Full term runs from 39 weeks through 40 weeks 6 days. Late term covers 41 weeks through 41 weeks 6 days, and postterm begins at 42 weeks. These distinctions matter because babies born even a week or two early can have slightly higher rates of breathing and feeding difficulties compared to those born at 39 weeks or later.
The Fourth Trimester
You may also hear about a “fourth trimester,” which refers to the first 12 weeks after birth. It’s not a clinical pregnancy stage, but the term captures the reality that major physical and emotional changes continue well beyond delivery. During those 12 weeks, your body is recovering from either vaginal tearing or a surgical incision, hormone levels are shifting dramatically, and sleep deprivation is at its peak. Many people are also navigating the physical and emotional learning curve of breastfeeding. Recognizing this period as its own distinct phase helps set realistic expectations for recovery.