What is a trimester?
A trimester is one of three roughly equal stages of a pregnancy, each about 13 to 14 weeks long. The first trimester runs from week 1 to the end of week 13, the second from week 14 to week 27, and the third from week 28 until birth. Weeks are counted from the first day of the last period.
The three trimesters at a glance
| Trimester | Weeks | What is happening |
|---|---|---|
| First | 1 to 13 | conception (around week 2), implantation, the embryo forms organs; nausea and tiredness are common |
| Second | 14 to 27 | growth, first movements felt around 16 to 22 weeks; many people feel more energetic |
| Third | 28 to birth | rapid weight gain for the baby, more discomfort, preparation for labor around week 40 |
Why weeks start before conception
Pregnancy is dated from the first day of your last menstrual period, about two weeks before you actually conceive. So at "4 weeks pregnant" the embryo is only around two weeks old. Your due date is 40 weeks from that first day, or adjusted by an early ultrasound. Try the pregnancy calculator to see which week and trimester you are in.
Mood across the trimesters
Hormone levels rise steeply in early pregnancy, and many people feel tearful, anxious or flat in the first trimester, then steadier in the second. Low mood or anxiety that lasts two weeks or more deserves a conversation with your midwife or doctor at any stage. Perinatal depression is common and treatable.
Get urgent care in any trimester for heavy bleeding, severe abdominal pain, fluid leaking, a severe headache with vision changes, or fewer baby movements later in pregnancy.
Frequently asked questions
Is week 13 first or second trimester?
Most sources count week 13 as the end of the first trimester and week 14 as the start of the second. Some put the cut-off at week 12.
What is the fourth trimester?
An informal name for the first 12 weeks after birth, when the body recovers and the cycle slowly returns. See postpartum period.
Sources
This page is for information only and isn鈥檛 medical advice. If something worries you, please talk to a doctor.
Keep reading
Pregnancy calculator: how far along am I?Pregnancy calculator: enter your last period or due date to see how many weeks and days pregnant you are, your trimester and the days to go. Free and private.Due date (EDD): definition and meaningYour estimated due date (EDD) is 40 weeks after your last period started. How it's calculated, why few babies arrive on it, and how ultrasound changes it.Human chorionic gonadotropin (hCG): definitionhCG is the hormone made after implantation that pregnancy tests detect. How fast it rises, what beta hCG numbers mean, and when a test can turn positive.
Due date calculator: when is my baby due?Free pregnancy due date calculator: enter the first day of your last period and cycle length to see your estimated due date and how many weeks along you are.
Postpartum period: definition and meaningThe postpartum period is the time after birth as the body recovers, often counted as 6 to 12 weeks. Bleeding, returning periods, and mood after birth.