Your Pregnancy Week by Week: What to Expect

First Trimester: Weeks 1-13

The first trimester is the most critical period of fetal development — all major organ systems form during this time. It is also the trimester with the highest risk of miscarriage, which is why many people wait until 12-13 weeks before sharing their news widely.

Weeks 1-4: Conception and Implantation

Weeks 1-2 are technically "before pregnancy" in terms of gestational age — they cover the menstrual period and ovulation. Fertilization typically occurs around day 14 (for a 28-day cycle). The fertilized egg (zygote) begins dividing as it travels down the fallopian tube, becoming a blastocyst by day 5-6. Implantation into the uterine wall occurs between days 6-10 after fertilization (roughly weeks 3-4 of gestational age).

At this stage: the embryo is smaller than a poppy seed. hCG begins being produced by the implanting blastocyst — this is what pregnancy tests detect. Some women notice light spotting (implantation bleeding) around the expected time of their period.

Weeks 5-8: Organogenesis Begins

This is the critical period when major organ systems begin forming. The neural tube (which becomes the brain and spinal cord) forms and closes by week 6, which is why adequate folic acid before conception is so important. The heart begins beating around week 5-6 and is often visible on transvaginal ultrasound by week 6.

Key developments: limb buds appear at week 5-6, growing into arm and leg paddles with digit formation by week 8. The face begins to form — eyes, nose, and mouth become identifiable. All major organs (heart, lungs, kidneys, liver) are present in primitive form by the end of week 8.

What you may feel: nausea peaks between 8-10 weeks and can be debilitating for some women. Extreme fatigue, breast tenderness, and frequent urination are common. Blood volume is expanding significantly.

Baby's size: approximately 0.08 inches at week 5 growing to about 0.63 inches (blueberry) at week 8.

Weeks 9-13: First Trimester Completion

By week 10, the embryo transitions to "fetus" — all organ systems are established and the next 30 weeks are primarily about growth and maturation. Fingers and toes are formed, bones begin hardening, and the external genitalia are developing (though too small to see on ultrasound at this stage).

Key milestones: the first prenatal appointment typically occurs at 8-10 weeks with bloodwork (blood type, Rh factor, CBC, rubella immunity, sexually transmitted infections screening). The nuchal translucency (NT) scan occurs at 11-13+6 weeks and measures fluid at the back of the fetal neck — an increased measurement can indicate chromosomal conditions. NIPT (cell-free fetal DNA) blood testing is available from 10 weeks onward for chromosomal screening.

By week 13, miscarriage risk drops substantially as organogenesis is complete. Most women find nausea improving. Baby's size: approximately 3 inches (lemon) at 13 weeks.

Second Trimester: Weeks 14-27

The second trimester is often called the "honeymoon trimester" — for many women, energy returns, morning sickness resolves, and the bump becomes visible while remaining manageable. It is also when you will hear the heartbeat more reliably and see your baby clearly for the first time at the anatomy scan.

Weeks 14-17: Energy Returns

Progesterone levels stabilize as the placenta takes over from the corpus luteum (at approximately 10-12 weeks). For many women, fatigue and nausea improve dramatically. The uterus begins to move above the pelvic brim, and the "bump" becomes visible externally.

Fetal development: the fetus is actively moving (though not yet detectable to the mother), with coordinated arm and leg movements. Taste buds form; the fetus begins swallowing amniotic fluid. Facial expressions — grimacing, yawning, squinting — are visible on high-resolution ultrasound. Size at 16 weeks: approximately 4.5 inches (avocado).

Weeks 18-22: Quickening and the Anatomy Scan

Quickening — the first sensation of fetal movement — typically occurs between 16-25 weeks. First-time mothers usually feel it between 18-22 weeks; experienced mothers often feel it earlier (16-18 weeks). The initial sensations are often described as flutters, bubbles, or light taps.

The anatomy scan (also called the 20-week scan or mid-pregnancy ultrasound) occurs between 18-22 weeks. This detailed ultrasound examines fetal anatomy head-to-toe, checking brain, heart, spine, kidneys, limbs, and placenta position. It is the primary screening tool for structural abnormalities. Gender can usually be determined at this scan. If the placenta is low-lying (placenta previa), follow-up imaging is scheduled.

Weeks 23-27: Viability Milestone

Week 24 is sometimes called the "viability threshold" — the gestational age at which survival with intensive NICU support becomes possible, though outcomes at this age remain challenging. By week 26-27, survival rates with NICU support exceed 80% at specialized centers.

Fetal development: the lungs begin producing surfactant — the substance that prevents alveoli from collapsing — though production is still insufficient for sustained independent breathing. The brain is developing rapidly. The fetus responds to sound and may startle at loud noises. Size at 24 weeks: approximately 12 inches and 1.3 pounds (corn).

For the mother: glucose screening (gestational diabetes test) typically occurs at 24-28 weeks. Braxton Hicks contractions (practice contractions) may begin, typically painless and irregular.

Third Trimester: Weeks 28-40

The third trimester is about preparation — the fetus grows rapidly, accumulating fat and maturing organ systems, while you prepare physically and logistically for birth.

Weeks 28-32: Rapid Growth

The fetus nearly doubles in weight between weeks 28-36. Lanugo (fine downy hair that covers fetal skin) begins disappearing as fat deposits develop under the skin. The eyes open and close. Brain development accelerates — the brain quadruples in weight in the third trimester.

RhoGAM injection is given at approximately 28 weeks for Rh-negative mothers. Group B Streptococcus (GBS) vaginal swab test occurs at 35-37 weeks. Size at 28 weeks: approximately 14.8 inches and 2.2 pounds (eggplant).

Weeks 33-36: Lung Maturation

Lung maturation — specifically surfactant production — intensifies between 33-36 weeks. By 34-35 weeks, most babies do not require respiratory support if born. The baby typically settles into a head-down position (cephalic presentation) during this period, though some remain breech until closer to term or beyond.

Braxton Hicks contractions may become more frequent and noticeable. The fundus (top of the uterus) reaches its highest point — just below the ribcage — before the baby "drops" (lightening) into the pelvis, typically in the last few weeks for first-time mothers.

Weeks 37-40: Full Term and Birth Preparation

ACOG defines 39 weeks as "full term" — the optimal gestational age for elective delivery when there are no medical indications for earlier birth. Babies born at 37-38 weeks (early term) have higher rates of complications than those reaching 39 weeks.

What to watch for: regular contractions (every 5 minutes, lasting 60 seconds, for 1 hour — the "5-1-1 rule"), water breaking, or significant decrease in fetal movement (which should prompt immediate contact with your provider).

Most providers recommend counting fetal kicks daily from 28 weeks — a healthy baby should move at least 10 times in 2 hours. Any sudden decrease in movement warrants immediate evaluation.

Size at 40 weeks: approximately 20 inches and 7.5 pounds (small pumpkin) — though individual babies vary considerably from these averages.