Pregnancy Trimester Calculator

The Three Trimesters of Pregnancy

Pregnancy is divided into three trimesters of approximately 13 weeks each, with distinct developmental milestones, common symptoms, and prenatal screening tests in each phase.

First Trimester (Weeks 1–13)

The most critical period for fetal development — all major organs and body systems form. The embryo is most vulnerable to toxins and nutritional deficiencies. Key screenings: 8–12 week ultrasound, first-trimester NIPT/nuchal translucency screening.

Second Trimester (Weeks 14–27)

The "golden trimester" — nausea typically fades, energy returns, and the baby bump becomes visible. Baby begins moving around weeks 18–22. Key screenings: anatomy ultrasound (18–22 weeks), glucose tolerance test (24–28 weeks).

Third Trimester (Weeks 28–40)

Baby gains most of its weight here, growing from ~2.5 lbs to ~7.5 lbs. Common symptoms: Braxton Hicks, back pain, heartburn, frequent urination. Key milestones: Group B Strep test (35–37 weeks), weekly OB visits from week 36.

What Prenatal Care Looks Like Trimester by Trimester

Prenatal visit frequency generally follows your trimester: monthly visits through the first and second trimesters, shifting to every 2 weeks starting around week 28, then weekly from week 36 until delivery. This isn't arbitrary — it reflects how much can change in later pregnancy and how much closer monitoring benefits both you and the baby as your due date approaches. Each trimester also comes with its own standard round of screenings, from early bloodwork and first-trimester genetic screening, to the mid-pregnancy anatomy scan, to the glucose tolerance test and Group B Strep screening later on.

Nutrition Needs Shift by Trimester

Contrary to the old "eating for two" idea, calorie needs actually increase only modestly during pregnancy — roughly no extra calories needed in the first trimester, about 300 extra per day in the second, and about 450 extra per day in the third. What matters more than quantity is the quality and consistency of nutrients: folate and iron are especially critical early on for neural tube development, calcium and vitamin D become increasingly important as the baby's skeleton develops in the second and third trimesters, and iron needs rise further in the third trimester to support increased blood volume.

Common Symptoms by Trimester and When They're Worth a Call

• First trimester: Nausea, fatigue, and breast tenderness are typical. Severe, persistent vomiting that prevents keeping fluids down (hyperemesis gravidarum) warrants a call to your provider.
• Second trimester: Round ligament pain, mild back discomfort, and increased appetite are common. Severe abdominal pain, heavy bleeding, or reduced fetal movement after you've started feeling regular movement should be reported right away.
• Third trimester: Braxton Hicks contractions, swelling, and back pain are expected. Sudden significant swelling with headaches or vision changes (possible signs of preeclampsia), regular contractions before 37 weeks, or a noticeable drop in fetal movement should be evaluated promptly.

How Trimester Timing Can Shift Slightly

You'll sometimes see trimester boundaries defined slightly differently between sources — some define the first trimester as weeks 1-12, others 1-13, and similar small variations for the second and third. This isn't inconsistency so much as trimesters being a useful organizing concept rather than a rigid medical boundary; your care team focuses more on your specific gestational week and what's developmentally expected at that point than which exact trimester label applies.

About This Tool: Method, Limits & Sources

What it does. Shows which trimester you are in and how long remains before the next one begins.

How it works. Completed weeks since the first day of the last period: first trimester up to week 13, second trimester weeks 14–27, third trimester week 28 onward. Days left = days until the end of week 13, 27 or 40 (the tool's boundaries).

Example. At 20 weeks you are in the second trimester; the tool counts the days left to the end of week 27, which is 49 days from week 20.

Limitations

• Trimester boundaries are conventions and vary slightly between sources.
• Accuracy depends on the last period date or ultrasound dating.

When to Speak With a Healthcare Professional

• You have heavy bleeding, severe abdominal pain, fever, or decreased fetal movement — seek urgent care
• You want to confirm your due date — your provider usually does this with an ultrasound
• You have any concern about your pregnancy

References

• ACOG Committee Opinion 700 — Methods for Estimating the Due Date

Last updated: October 2026 · Published by Bloom Health Woman (not medically reviewed) · Editorial Policy