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First Trimester
Weeks 1–13
What's Happening With Baby
Even though there's nothing to see on the outside yet, this is one of the most important stretches of the whole pregnancy — nearly all of baby's major organs begin forming in these 13 weeks.
- Weeks 1–8: A fertilized egg implants in the uterus and becomes a blastocyst. By week 5, the embryo stage begins — the brain, spine, and neural tube start forming, followed by early cardiac tissue and the beginnings of the face and inner ear.
- By the end of the first trimester: Baby is roughly the size of a lemon, with all major organ systems established in early form.
Source: ACOG, "Changes During Pregnancy"; American Academy of Family Physicians, "Your Baby's Development: The First Trimester"
What's Happening In Your Body
Fatigue and nausea are the two symptoms almost everyone mentions first — and for good reason.
- Nausea ("morning sickness") affects an estimated 70–80% of pregnancies, largely linked to rising hCG levels. Despite the name, it can happen any time of day.
- Fatigue ranks just as high. Nobody knows exactly why, but the rapid rise in progesterone in early pregnancy is thought to play a major role.
- Sore, swollen breasts — your body is already starting to prepare for feeding.
- Frequent urination, thanks to increased blood volume sending more fluid through your kidneys.
- Moodiness, bloating, food aversions, and a heightened sense of smell are all common and considered normal hormonal responses.
Source: Mayo Clinic, "Symptoms of Pregnancy: What Happens First"; ACOG
Your Prenatal Care in the First Trimester
- Call to schedule your first appointment as soon as you get a positive test — it's often booked further out than you'd expect.
- Your first visit is typically the longest of the pregnancy (around 45 minutes) and usually happens between weeks 6–8. Expect a full personal and family medical history review, confirmation of your due date, and often an early ultrasound — sometimes transvaginal at this stage — to check the fetal heartbeat and size, and to check for multiples.
- Follow-up visits in the first trimester are typically shorter, and you can expect two to three total before moving into the second trimester.
- Genetic screening can be offered starting around week 10, if you choose it.
Source: Mayo Clinic, "Prenatal Care: First Trimester Visits"; Cleveland Clinic, "First Trimester of Pregnancy"
When to Call Your Provider
Most first-trimester symptoms are uncomfortable but expected. These are worth a call, not a wait-and-see:
- Vaginal bleeding or spotting
- Severe or persistent cramping
- Severe nausea/vomiting that prevents you from keeping any food or fluids down
- Colored (green/yellow) or foul-smelling discharge
- Pain when urinating (possible UTI)
- Fever above 100.4°F
Source: UPMC, "Your Guide to the First Trimester"
FAQs
When does the first trimester officially end?
ACOG defines it as ending after week 13 — you're officially in the second trimester once you hit 14 weeks, 0 days.
Is it normal to not feel pregnant at all?
Yes. Symptoms vary enormously between people and even between pregnancies for the same person. A quiet first trimester isn't a warning sign by itself.
When should I start buying baby gear?
There's no rush. Most providers and parenting resources suggest using this trimester to learn and save ideas rather than shop — most purchases naturally happen later once you have more clarity (and less nausea).
"You don't have to look pregnant to be doing the hardest, most invisible work of the whole nine months."