First trimester: how many weeks, and what happens
By Pregora Editorial Team · Updated 2026-08-22 · 9 min read
The first trimester runs from week 1 to the end of week 13, so it closes the day you reach 14 weeks. That is roughly three months, though not quite, because a month is 4.33 weeks rather than four.
The odd part is where the counting starts. Weeks 1 and 2 sit before conception, which means the first trimester is measured from a point at which you were not pregnant at all.
Where the boundaries actually fall
| Trimester | Weeks | Roughly |
|---|---|---|
| First | 0 to 13 weeks and 6 days | Months 1 to 3 |
| Second | 14 to 27 weeks and 6 days | Months 4 to 6 |
| Third | 28 weeks to birth | Months 7 to 9 |
You will occasionally see the first trimester quoted as ending at 12 weeks. Both figures circulate, which is why the answer feels slippery. Clinically the trimesters are defined in weeks, and the second begins at 14 weeks and 0 days, so the first covers everything up to 13 weeks and 6 days.
The trimester calculator works out which trimester you are in and how many days remain in it, from your last period or a scan date.
Why weeks 1 and 2 come before conception
Gestational age is counted from the first day of your last menstrual period, not from conception. On day one you were menstruating. Ovulation follows around two weeks later in a 28-day cycle, and conception with it.
The convention exists because the date of a period is something almost everyone can name, while the hour of conception is not. The system standardised on the measurable thing. The consequence is that when you are told you are six weeks pregnant, the embryo is closer to four weeks old. We cover that gap in more depth in how a due date is calculated.
The milestones inside the first trimester
Everything below is in gestational weeks, counted from your last period.
- Weeks 1 to 2. Menstruation, then the follicular phase. Not pregnant yet.
- Week 3. Ovulation and fertilisation in a textbook cycle. Later if your cycles run long.
- Week 4. Implantation completes and hCG begins to rise. This is when a period is missed and a test turns positive.
- Weeks 5 to 6. A gestational sac becomes visible on a transvaginal scan, and cardiac activity is often detectable from around six weeks.
- Week 8. The embryonic period ends. From here the term used is fetus rather than embryo.
- Week 10. NIPT blood screening becomes available.
- Weeks 11 to 14. The dating scan, often combined with nuchal translucency screening. This is the measurement that sets your due date for the rest of the pregnancy.
- Week 12. Miscarriage risk falls markedly, which is why many people wait until now to share the news.
- Week 13. The trimester ends.
What tends to be hardest
Nausea affects the majority of pregnancies to some degree. It typically begins around six weeks, is at its worst near nine weeks, and eases for most people between twelve and fourteen weeks. The name is unhelpful, since it is not confined to mornings.
Fatigue in the first trimester is frequently described as unlike ordinary tiredness, and it is doing real work: the placenta is being built. Breast tenderness, needing to urinate more often, food aversions and a sharpened sense of smell are all common and all driven by the same hormonal shift.
Most of it improves in the second trimester, which is the stretch people generally find easiest.
Miscarriage risk, stated plainly
Roughly one in ten to one in seven clinically recognised pregnancies ends in miscarriage, and the large majority of those happen in the first trimester. Most are caused by chromosomal errors that occurred at fertilisation and could not have been prevented by anything the pregnant person did or did not do.
Risk drops substantially once a heartbeat has been seen and again after twelve weeks. Bleeding is not automatically a sign of loss either. Around a quarter of first trimesters involve some bleeding and most of those pregnancies continue, which we go through in can you be pregnant and still have a period.
The one thing worth starting immediately
Folic acid. The neural tube, which becomes the brain and spinal cord, closes very early, within roughly the first six weeks of gestational age. That is often before a pregnancy is confirmed, which is why the standard advice is 400 micrograms daily from before conception and through the first trimester.
If you are reading this having just found out, starting now is still worth doing. Some people are advised a higher dose for medical reasons, so ask rather than assume.
Contact your midwife, doctor or an early pregnancy unit straight away if you have heavy bleeding, severe or one-sided pain, pain in the tip of your shoulder, a fever, or vomiting so persistent that you cannot keep fluids down.
Frequently asked questions
How many weeks is the first trimester?
Fourteen weeks of counting, covering weeks 0 through 13 and 6 days. The second trimester starts the moment you reach 14 weeks and 0 days.
When does the first trimester end, 12 weeks or 13?
Thirteen weeks and six days. The 12-week figure gets repeated because the dating scan often happens around then and because miscarriage risk drops at that point, but the trimester boundary itself sits at 14 weeks.
Is the first trimester three months?
Approximately. Fourteen weeks divided by 4.33 weeks per month is about 3.2 months. Assuming four weeks to a month is where the arithmetic goes wrong and why pregnancy sometimes gets described as ten months long.
Why does my scan date differ from my own calculation?
Almost always because ovulation happened later than day 14. Ovulation sits about 14 days before your next period rather than 14 days after your last, so longer cycles push it back. Between 8 and 13 weeks an ultrasound is accurate to within about five to seven days, so where it disagrees by more than a week, the scan is taken as the date.
Disclaimer: General education, not medical advice. Screening offered, scan timing and supplement doses vary by country and by individual history. Follow the guidance of your own midwife or doctor.