Pregnancy Trimester Guide

First Trimester (Weeks 1–12): Formation

The first trimester is the most developmentally intense period of pregnancy. By week 6, the embryo has a beating heart — typically 90–110 beats per minute on transvaginal ultrasound. By week 10, all major organ systems have formed in their basic structure: the brain, spinal cord, heart, kidneys, liver, and limb buds are all present. The embryo becomes a fetus at around 10 weeks, marking the transition from organ formation to organ refinement.

Miscarriage risk is highest in the first trimester — approximately 10–15% of clinically recognized pregnancies, and higher when very early losses are included. The risk drops substantially after cardiac activity is confirmed at 8–10 weeks and again after 12 weeks. First-trimester screening (nuchal translucency ultrasound + blood tests at 11–13 weeks) can assess the risk of chromosomal conditions including Down syndrome (trisomy 21), trisomy 18, and trisomy 13.

Nausea and vomiting affect approximately 70–80% of pregnant people in the first trimester (ACOG, 2018). Symptoms typically peak at 8–10 weeks and improve by 14–16 weeks for most, though a small percentage experience hyperemesis gravidarum — severe nausea and vomiting requiring medical treatment.

Second Trimester (Weeks 13–27): Growth and Movement

The second trimester is often the most comfortable. The fetus grows from roughly 3 inches at 13 weeks to about 14 inches at 27 weeks. Fetal movements (quickening) are typically felt between 16 and 22 weeks — earlier in subsequent pregnancies, later in first pregnancies and with an anterior placenta. By 24–25 weeks, the fetus reaches the threshold of viability with intensive neonatal support, though survival outcomes improve significantly with each additional week of gestation.

The anatomy scan (typically at 18–22 weeks) is the most comprehensive structural ultrasound of pregnancy. It evaluates fetal anatomy across 28+ standard measurements including brain structure, cardiac chambers, spine, abdominal organs, limbs, and placental position. ACOG recommends this scan for all pregnancies (ACOG, 2016). Gestational diabetes screening occurs at 24–28 weeks via the one-hour glucose challenge test, with a three-hour glucose tolerance test for those who screen positive.

Third Trimester (Weeks 28–40+): Preparation and Delivery

The third trimester is defined by rapid weight gain — the fetus gains approximately half a pound per week in the final month — and preparation for life outside the womb. Lung maturation is the key developmental milestone: surfactant production, which allows the lungs to expand and contract without collapsing, matures at approximately 34–36 weeks. This is why late-preterm birth (34–36 weeks) carries different risks than early-term birth (37–38 weeks) or full-term birth (39–40 weeks).

Group B Streptococcus (GBS) screening occurs at 36–37 weeks. GBS colonizes the vaginal and rectal area in approximately 10–30% of pregnant people and can cause serious infections in newborns during delivery. ACOG recommends universal GBS screening and intrapartum antibiotic prophylaxis for those who test positive (ACOG, 2020). After 37 weeks, fetal movement monitoring becomes important — a noticeable decrease in movement from your established pattern warrants same-day contact with your provider.

Prenatal appointments shift to every two weeks at 28–36 weeks, then weekly from 36 weeks onward. ACOG defines full-term as 39–40 weeks and recommends against elective delivery before 39 weeks due to higher rates of NICU admission, respiratory complications, and breastfeeding difficulties in early-term infants.