GUIDE
First Trimester Guide
Published on ·Updated on
Everything you need to know about weeks 1 through 13 — from conception to the end of the highest-risk period.
The first trimester is when your baby develops from a single fertilized egg into a fully formed fetus with all major organs in place. It is also the period when pregnancy symptoms tend to be most intense and the risk of miscarriage is highest. By the end of week 13, most of that risk has passed and you are heading into the more comfortable second trimester.
Be prepared from day one
“There's a host of reasons why prenatal care is important. The earlier that we can get patients seen, the earlier we can start interventions that can improve these longer-term outcomes.”
What Happens in the First Trimester
The first trimester spans weeks 1 through 13 and is the most transformative period of pregnancy. In just 13 weeks, a single fertilized cell develops into a fully formed fetus with a beating heart, functioning organs, and the ability to move, swallow, and respond to touch.
For you, this period is often the most physically challenging. Rising levels of hCG and progesterone drive the symptoms that define early pregnancy — morning sickness, fatigue, breast tenderness, and food aversions. These symptoms tend to peak around weeks 8 to 10 and gradually improve as you approach the second trimester.
The first trimester is also when your foundational prenatal care is established. Your first prenatal visit typically happens between weeks 6 and 10, and early screening tests like the dating ultrasound and NIPT help your provider monitor the pregnancy from the start.
If you are just finding out you are pregnant, see our guide on early signs of pregnancy and make sure you are taking prenatal vitamins.
| Week | Baby Size | Key Development |
|---|---|---|
| Week 1 | — | Cycle begins — pregnancy is dated from the first day of your last menstrual period |
| Week 2 | — | Ovulation and conception occur near the end of this week |
| Week 3 | Poppy seed | Fertilization — the blastocyst implants into the uterine lining |
| Week 4 | Poppy seed | Implantation complete — hCG production begins, missed period expected |
| Week 5 | Sesame seed | Heart tube forms and begins beating — neural tube developing |
| Week 6 | Lentil | Heartbeat detectable on ultrasound — facial features forming |
| Week 7 | Blueberry | Brain growing rapidly — arms and legs budding |
| Week 8 | Raspberry | All major organs forming — fingers and toes beginning to develop |
| Week 9 | Cherry | Embryo becomes a fetus — external genitalia forming |
| Week 10 | Strawberry | Vital organs functioning — fingernails and bones hardening |
| Week 11 | Lime | Baby can swallow and kick — tooth buds forming |
| Week 12 | Plum | Reflexes developing — miscarriage risk drops significantly |
| Week 13 | Lemon | Vocal cords forming — fingerprints becoming unique |
Month-by-Month Breakdown
The first trimester covers roughly the first three months of pregnancy. Each month brings distinct changes.
During month 1 (weeks 1-4), pregnancy begins before you even know it. Conception, implantation, and the very earliest cell division happen during this time. Most people discover they are pregnant near the end of this month when they miss a period.
At month 2 (weeks 5-8), symptoms hit hardest. Morning sickness typically peaks, the heartbeat becomes visible on ultrasound, and you will likely have your first prenatal visit. All major organs begin forming during this critical period.
By month 3 (weeks 9-13), the embryo is reclassified as a fetus, miscarriage risk drops significantly, and first-trimester screening wraps up. Many people share their pregnancy news after reaching this milestone.
Common First Trimester Symptoms
- Nausea and vomiting (morning sickness) — typically peaks weeks 8-10
- Extreme fatigue — progesterone levels surge in early pregnancy
- Breast tenderness and swelling
- Frequent urination as the uterus presses on the bladder
- Food aversions and heightened sense of smell
- Bloating and constipation from slowed digestion
- Mood swings from hormonal fluctuations
- Light spotting or cramping around implantation (weeks 3-4)
- Headaches and dizziness
Symptom severity varies widely. Some people experience all of these while others have very few. Both are normal.
Key Tests and Screenings
The first trimester includes several important prenatal tests and screenings. Your provider will guide you through which ones are recommended for your situation.
The dating ultrasound (weeks 6-10) confirms the pregnancy location, checks for a heartbeat, and establishes your due date. This is typically done at your first prenatal visit, which also includes comprehensive blood work and a physical exam.
From week 10, the NIPT (non-invasive prenatal testing) is available. This blood test screens for common chromosomal conditions like Down syndrome with high accuracy. It can also reveal the baby's sex if you want to know.
Between weeks 11 and 13, a nuchal translucency scan may be offered as part of first-trimester combined screening. This ultrasound measures the fluid at the back of the baby's neck to assess risk for certain chromosomal conditions.
Week-by-Week Guides
For detailed information about each week in the first trimester, explore the individual guides below.
Week 1 and Week 2 cover the pre-conception period. Week 3 is when fertilization typically occurs. Week 4 brings implantation and your missed period.
Week 5 is when the heart begins beating. Week 6 is often when the heartbeat is first seen on ultrasound. Week 7 and Week 8 are peak symptom weeks with rapid brain and organ development.
Week 9 marks the transition from embryo to fetus. Week 10 is the NIPT window opening. Week 11 and Week 12 bring improving symptoms and declining miscarriage risk. Week 13 closes out the first trimester.
When to Call Your Doctor
- Heavy vaginal bleeding soaking a pad per hour
- Severe abdominal or pelvic pain, especially one-sided
- Fever above 100.4 degrees Fahrenheit
- Inability to keep any food or fluids down for 24 hours
- Painful urination — UTIs need treatment during pregnancy
- Dizziness or fainting episodes
When in doubt, call your provider. Early pregnancy concerns are always worth a call.
Looking Ahead to the Second Trimester
The second trimester (weeks 14-27) is often called the honeymoon phase of pregnancy. Morning sickness typically fades, energy returns, and you will start to feel your baby move. The anatomy scan around week 20 provides your most detailed look at the baby yet.
If you are in the thick of first-trimester symptoms, hang in there — relief is coming. For managing specific symptoms now, see our guides on morning sickness and pregnancy fatigue.
Related Guides
Sources
- American College of Obstetricians and Gynecologists (ACOG) — Pregnancy FAQ
- Mayo Clinic — Fetal development: trimester by trimester
- March of Dimes — Pregnancy week by week
- National Institutes of Health (NIH) — What are the stages of pregnancy?
This guide is for informational purposes only and is not a substitute for professional medical advice. Always consult your healthcare provider with any questions about your pregnancy.
Frequently asked questions
- When does the first trimester start and end?
- The first trimester begins on the first day of your last menstrual period (LMP) and ends at 13 weeks 6 days. This means the first two weeks of the first trimester occur before conception has even happened — pregnancy is dated from the LMP, not from fertilization. The embryonic period (when organs form) spans roughly weeks 3 through 10, and by week 11 the embryo is reclassified as a fetus.
- Is morning sickness a good sign in the first trimester?
- Morning sickness is associated with lower rates of miscarriage, but the absence of nausea does not mean anything is wrong. About 20-30% of people have healthy pregnancies without significant nausea. Morning sickness typically peaks between weeks 8 and 10 and improves by weeks 12 to 14. If you are unable to keep any food or fluids down, talk to your provider about hyperemesis gravidarum.
- What prenatal tests happen in the first trimester?
- Common first-trimester tests include a dating ultrasound (weeks 6-10) to confirm the pregnancy and establish your due date, initial blood work at your first prenatal visit, optional NIPT screening (from week 10) for chromosomal conditions, and a nuchal translucency scan (weeks 11-13). Your provider will recommend which tests are appropriate for your situation.
- When is the risk of miscarriage lowest in the first trimester?
- Miscarriage risk drops significantly after a heartbeat is confirmed on ultrasound (typically around weeks 6-8) and drops even further after week 12, falling to about 1-2%. The majority of first-trimester miscarriages occur before week 10 and are caused by chromosomal abnormalities that are beyond anyone's control.
