How Many Weeks Pregnant Am I?

How pregnancy weeks are counted

Pregnancy is measured in gestational age, counted from the first day of your last menstrual period (LMP), even though conception happens about two weeks later. The LMP works as the starting point because it's a concrete, observable date, while the exact day of ovulation and fertilization is rarely known. Your provider, ultrasound technician, and every prenatal screening guideline use gestational age by default.

So at "4 weeks pregnant" (gestational age), the embryo itself is about 2 weeks old, but your provider will always call you 4 weeks along. Prenatal care guidelines, screening windows, and fetal development charts are all written this way.

Gestational age vs. fetal age

Gestational age counts from the LMP and is the universal clinical standard. Fetal age (or embryonic age) counts from fertilization and runs about two weeks less. Development descriptions in books or online, such as "at 8 weeks, the heart has four chambers," sometimes use fetal age, so check which one a source means.

With your provider, always use gestational age. When reading fetal development content, check whether weeks are counted from LMP or from conception.

First trimester: weeks 1–13

The first trimester is when the foundations form fast. By its end, the embryo has become a fetus with all major organ systems in place, a beating heart, and the beginnings of every limb and facial feature. The placenta has fully formed and taken over hormone production from the corpus luteum.

Common first-trimester experiences include nausea and vomiting (affecting 70–80% of pregnancies), fatigue, breast tenderness, frequent urination, and heightened sensitivity to smells. Most of these symptoms begin to improve by weeks 12–14.

Key first-trimester appointments: First prenatal visit (weeks 8–10); first-trimester screening including nuchal translucency ultrasound and blood draw (weeks 11–13); possible cell-free DNA (cfDNA) testing starting at week 10.

Second trimester: weeks 14–27

The second trimester is often the most comfortable phase. As the placenta fully takes over and hormones stabilize, nausea typically fades, energy returns, and the visible pregnancy bump appears. The fetus grows from about 3 inches and 1 ounce at week 14 to roughly 14 inches and 2 pounds by week 27.

Around weeks 18–22, most parents feel fetal movement for the first time. The anatomy scan, a detailed ultrasound checking fetal growth, organ development, and placental position, is scheduled in this window. If you're having genetic testing, second-trimester quad screen results usually come back in this period.

Key second-trimester appointments: Anatomy scan (weeks 18–22); quad screen if elected (weeks 15–20); glucose challenge test (weeks 24–28).

Third trimester: weeks 28–40+

In the third trimester, your baby gains most of their body weight and the lungs begin producing surfactant, the substance essential for breathing after birth. By week 37, a baby is "early term." By week 39, the brain, lungs, and liver are more fully mature, which is "full term."

Prenatal visits become more frequent (every two weeks from weeks 28–36, then weekly from week 36). Your provider will discuss your birth preferences, check for signs of preeclampsia, and monitor the baby's position.

Key third-trimester appointments: Biweekly visits (weeks 28–36); Group B Strep test (weeks 35–37); weekly visits (week 36 onward); non-stress tests if indicated; discussion of induction if approaching 41–42 weeks.

Keep exploring

Frequently asked questions

Sources & further reading

1. ACOG Committee Opinion 700: Methods for Estimating the Due Date
2. ACOG FAQ: How Your Baby Grows During Pregnancy
3. MedlinePlus: Fetal Development
4. ACOG FAQ: Prenatal Genetic Screening Tests