First Trimester Guide: Weeks 1-13
The first trimester is a period of rapid change. By the end of week 13, your baby's major organs have formed, and you may be starting to feel the first real symptoms of pregnancy.
What Happens in the First Trimester
Weeks 1-4: The Hidden Weeks
During the first four weeks, you may not know you are pregnant. Fertilization occurs around week 2, and implantation around week 3-4. Many women have no symptoms at all during this period.
Weeks 5-8: Early Symptoms Begin
This is when most women first notice pregnancy symptoms:
- Missed period
- Breast tenderness
- Nausea (with or without vomiting)
- Fatigue
- Frequent urination
- Food aversions or cravings
Weeks 9-13: Development Accelerates
By the end of the first trimester:
- All major organs have formed
- The heart is beating
- Fingers and toes are visible
- The baby is about 3 inches long
First-Trimester Screening
Nuchal Translucency (NT) Scan (11-13 weeks)
The NT scan measures fluid at the back of the baby's neck. Combined with blood tests (PAPP-A and hCG), it assesses the risk of chromosomal conditions including Down syndrome.
Cell-Free DNA Testing (10+ weeks)
A blood test that analyzes fetal DNA circulating in the mother's blood. It can screen for chromosomal conditions and, in some cases, determine the baby's sex.
Common First-Trimester Symptoms
Nausea and Vomiting
Often called "morning sickness," nausea can occur at any time of day. For most women, it peaks around weeks 9-10 and improves by weeks 14-16.
When to call your clinician: If you cannot keep any food or liquids down for 24 hours, you may be at risk for dehydration.
Fatigue
Extreme tiredness is common in the first trimester due to rising progesterone levels. Your body is working hard to support the pregnancy.
Breast Changes
Breasts may feel tender, swollen, or tingly. The areolas may darken, and small bumps (Montgomery tubercles) may become more visible.
Frequent Urination
Growing pressure from the uterus on the bladder causes more frequent trips to the bathroom. This usually improves in the second trimester as the uterus rises out of the pelvis.
What to Avoid in the First Trimester
- Alcohol: No amount of alcohol is known to be safe during pregnancy.
- Smoking: Smoking increases the risk of miscarriage, preterm birth, and low birth weight.
- Raw or undercooked meat: Risk of toxoplasmosis and listeria.
- High-mercury fish: Avoid shark, swordfish, king mackerel, and tilefish.
- Unpasteurized dairy: Risk of listeria.
- Caffeine: Limit to 200mg per day (about one 12-ounce cup of coffee).
Your First Prenatal Visit
Most providers schedule the first prenatal visit between weeks 6-8. At this visit, you can expect:
- Confirmation of pregnancy
- Medical history review
- Physical exam
- Blood tests (blood type, Rh factor, anemia, infections)
- Urine tests
- Discussion of prenatal vitamins
Prenatal Vitamins and Supplements
A prenatal vitamin helps fill nutritional gaps during pregnancy. Key nutrients include:
- Folic acid: 400-800 mcg daily — reduces the risk of neural tube defects
- Iron: 27 mg daily — supports increased blood volume
- Calcium: 1,000 mg daily — supports bone development
- Vitamin D: 600 IU daily — supports bone health and immune function
- DHA: 200-300 mg daily — supports brain and eye development
Always discuss supplements with your clinician before starting them. Some supplements may interact with medications or may not be appropriate for your specific situation.
When to Call Your Clinician
Contact your healthcare provider immediately if you experience:
- Heavy vaginal bleeding (soaking a pad in an hour)
- Severe abdominal pain or cramping
- Fever over 100.4°F (38°C)
- Severe vomiting (cannot keep liquids down)
- Pain or burning during urination
- Sudden swelling of hands, face, or eyes
Related Tools
- Gestational Age Calculator — how many weeks pregnant are you?
- How Many Weeks Pregnant — understand the dating method
- Trimester Calculator — which trimester are you in?
- Pregnancy Milestone Dates — when is your NT scan?