Pregnancy Week by Week: Your Complete Trimester-by-Trimester Guide

If you just found out you're pregnant — or you're trying to figure out exactly where you stand in the 40-week journey ahead — you're in the right place. "Pregnancy week by week" is one of the most searched phrases by expectant parents for good reason: understanding what each week actually involves turns a long, anxious road into a series of smaller, manageable chapters.

I'm Erin Grealy, a COPE-certified birth doula. This guide is the hub — a complete week-by-week pregnancy guide covering every single week from 1 through 40, the milestones that matter, the symptoms to expect, and the moments that deserve a call to your provider. For the interactive deep dive — where you can tap any week and see exactly what's happening to your baby and your body — see our interactive week-by-week tool. Not sure of your dates? Start with the due date calculator to anchor everything that follows. For a week-by-week preview of what comes after birth — physical healing, emotional shifts, and the fourth trimester — see our postpartum recovery guide.

How Pregnancy Weeks Work

Pregnancy is counted from the first day of your last menstrual period (LMP) — not from the day of conception. This is why week 1 and week 2 technically happen before there's even an egg to fertilize. Your body is simply preparing its uterine lining. Ovulation and fertilization typically occur around the end of week 2, and implantation happens by the start of week 4. From there, a full-term pregnancy runs about 40 weeks from your LMP, which is why "due dates" land roughly 38 weeks after conception.

If you're not sure of your LMP — or you have irregular cycles — an early dating ultrasound (usually done between 8 and 12 weeks) gives a much more accurate gestational age. Use our due date calculator for a quick estimate and read that post for the full breakdown of Naegele's rule, why only 4–5% of babies arrive on their exact due date, and how your provider may adjust the date after an ultrasound.

First Trimester: Weeks 1–13

The first trimester is the busiest 13 weeks in human development. In just a few months, a single fertilized cell divides into a tiny, recognizably human baby with a beating heart, all major organs, and the beginnings of facial features. Week 1–2: Your uterine lining is thickening in response to hormones; the egg isn't even fertilized yet. Week 3: Conception happens. The fertilized egg travels down the fallopian tube and begins dividing. Week 4: Implantation burrows into the uterine wall and hCG begins being produced — this is the hormone a pregnancy test is detecting when you see that positive line. For a walkthrough of those first days and what's happening biologically, see our first trimester symptoms guide.

Weeks 5–8 are when the work of organ-building happens. By week 6, a fetal heartbeat can usually be seen on ultrasound, often around 100–120 bpm. By week 7, arm and leg buds are visible, the brain is forming into five distinct areas, and the beginnings of the eyes, ears, and palate are present. By week 8, the embryo is officially a fetus, with every major organ system in rudimentary form. This is also the trimester with the highest risk of miscarriage — most of which is due to chromosomal factors outside anyone's control. Many parents choose to share their news after week 12, when that risk drops sharply.

Weeks 9–13 are when things start feeling more real — and more uncomfortable. The NIPT (non-invasive prenatal test) and the nuchal translucency scan usually happen in this window, screening for chromosomal conditions. By week 12, every major organ system is in place, and growth shifts from "formation" to "refinement." Around the same time, the worst of the hormone-driven symptoms — peak hCG, peak progesterone, peak nausea — start to ease for most people. You may notice a small bump beginning to show, and you'll likely have your first prenatal visit with full dating confirmation. For an honest, practical read on getting through these weeks, see our first trimester survival guide and the complete symptoms reference for what to expect, what's normal, and when something deserves a call.

Second Trimester: Weeks 14–27

Welcome to the trimester most parents describe as the "honeymoon" of pregnancy. Energy typically returns, nausea fades, and you finally look pregnant instead of just feeling exhausted. Weeks 14–17 bring the welcome return of appetite and the start of visible changes — a growing belly, the need for maternity clothes for most people, and the beginning of the emotional shift from "is this real?" to "this is real, and I'm doing this." Around week 16, baby's ears develop enough to begin responding to sound; many parents start talking, singing, or playing music around this time.

Weeks 18–22 are the headline weeks of the second trimester. The anatomy scan — a detailed ultrasound usually scheduled between week 18 and week 22 — checks baby's brain, heart, kidneys, limbs, spine, and placenta in fine detail. Most parents learn their baby's sex at this appointment if they want to. It's also when most people first feel fetal movement — gentle flutters around weeks 18–20, gradually strengthening into recognizable kicks and rolls. If you've been pregnant before, you may notice those flutters earlier, around weeks 16–18, simply because you know what to feel for. If you're not sure of your dates, the due date calculator post explains how your provider refines the LMP-based estimate with ultrasound dating.

Weeks 23–27 are when the third-trimester prep quietly begins. Around week 24, baby crosses the viability threshold — meaning babies born from this point on have a meaningful chance of survival with modern neonatal care. Around week 26–28, the standard glucose screen (the sugary-drink test for gestational diabetes) is usually scheduled. The brain is rapidly forming grooves and folds, lungs are developing surfactant that will eventually let them breathe room air, and your baby is putting on the fat stores that will help regulate temperature after birth. This is also when many parents start birth-prep seriously — drafting a birth plan, choosing a pediatric provider, and beginning conversations with a doula or midwife.

Third Trimester: Weeks 28–40

The third trimester is the home stretch — slower, heavier, and increasingly focused on the finish line. Baby is putting on roughly half a pound per week, lungs are maturing, and brain development continues at full pace. Weeks 28–32 mark the start of the third trimester and the start of daily kick counting. Most providers want you to track your baby's movement pattern from this point forward — use our kick counter to log patterns and know when reduced movement deserves a call. Around week 28, the RhoGAM shot is given if your blood type is Rh-negative. Around week 32, many parents get the Tdap vaccine (to protect baby from whooping cough after birth), and the Group B Strep test is typically scheduled between weeks 36 and 37.

Weeks 33–37 are when the third trimester becomes physically real. Back pain, pelvic pressure, frequent bathroom trips, and difficulty sleeping intensify — partly because baby has dropped lower into the pelvis (a process called "lightening") and partly because the body is genuinely preparing for labor. Around week 36, provider visits usually shift to weekly. Many people begin noticing colostrum — the first, nutrient-dense milk — leaking or expressible from the breasts around this time. This is also the right window to have your hospital bag packed and your hospital bag checklist complete (have it in the car by 36 weeks). Learn the signs of labor now so you're not guessing between real labor and Braxton Hicks at 2 a.m.

Weeks 38–40 are the final stretch — and those last weeks matter more than people realize. Babies born at 37–38 weeks are considered "early term," babies at 39–40 weeks are "full term," and those last two weeks involve rapid brain growth, lung maturation, and the development of the feeding and suck reflexes that make latching and breathing easier after birth. Most providers aim to support spontaneous labor until 41 weeks; induction is usually discussed between 41 and 42 weeks. Week 40 is your "estimated due date" — the midpoint of the wide normal range when babies actually arrive (most arrive between 39 and 41 weeks). Around week 38, many providers begin checking cervical effacement and dilation at the weekly visits. This is the right window to finalize your postpartum recovery plan, line up help for the first six weeks at home, and time contractions when labor starts.

Week-by-Week Symptoms by Stage

Most pregnancy symptoms cluster by stage rather than by individual week. Here's what most expecting parents experience in each trimester, and what to read for the deeper guide.

First trimester (weeks 1–13): Nausea (with or without vomiting), profound fatigue, breast tenderness, frequent urination, food cravings and aversions, mild pelvic cramping, bloating, headaches, mood swings, and the famous "pregnancy glow" — which is real, driven by increased blood volume. For a complete walkthrough of what's normal and when to call, see our first trimester symptoms guide.

Second trimester (weeks 14–27): Round ligament pain (sharp pulling sensations on either side of the lower belly, especially when rolling over or standing up), nasal congestion ("pregnancy rhinitis"), skin changes including the "linea nigra" down the abdomen, mild swelling of feet and ankles, vivid dreams, and — most excitingly — quickening, the first detectable fetal movements between weeks 16 and 22. Energy, mood, and appetite usually stabilize. For the full reference, see the complete symptoms guide.

Third trimester (weeks 28–40): Braxton Hicks contractions (irregular, non-rhythmic tightening that doesn't intensify), back pain, pelvic pressure, leg cramps, varicose veins, hemorrhoids, heartburn, shortness of breath when active, lightning crotch (sharp, brief nerve zings as baby settles lower), insomnia, and a near-constant need to urinate. Movement patterns become more defined — kicks, rolls, hiccups, and stretches — and counting them from week 28 onward is one of the most useful habits of late pregnancy. Cross-reference anything that feels off with our symptoms guide, and always call your provider when something feels genuinely wrong.

Key Milestones at a Glance

• Week 4: Implantation complete; positive home pregnancy test possible.
• Week 6: Fetal heartbeat visible on ultrasound (often 100–120 bpm).
• Week 8: Embryo officially becomes a fetus; all major organs present in rudimentary form.
• Week 10–12: hCG and progesterone peak; NIPT and nuchal translucency screening window.
• Week 12: End of highest miscarriage risk window; most early pregnancy symptoms start to ease.
• Week 16–18: First fetal movements (flutters) for experienced moms; weeks 18–25 for first-time moms.
• Week 18–22: Anatomy scan — baby's organs, spine, growth, and sex (if desired) are assessed in detail.
• Week 24: Viability threshold — baby has a reasonable chance of survival with intensive medical support if born now.
• Week 26–28: Glucose screen for gestational diabetes; brain rapid-growth window.
• Week 28: Third trimester begins; daily kick counting recommended; RhoGAM if Rh-negative.
• Week 32: Tdap vaccine usually given; baby's lungs maturing quickly.
• Week 36: Baby is considered "late preterm"; provider visits usually shift to weekly; hospital bag should be packed.
• Week 37–38: "Early term" — baby is considered developed; Group B Strep test scheduled.
• Week 39–40: "Full term" — most babies arrive in this window.
• Week 41–42: "Late term" and "post-term" — providers typically discuss induction between 41 and 42 weeks.

When to Call Your Provider

Most pregnancy symptoms are normal at some point — but some deserve an immediate call. Reach out to your OB or midwife for:

• Bleeding: Heavy vaginal bleeding (soaking a pad), especially with cramping or passage of clots
• Severe abdominal pain: Especially one-sided pain, with or without shoulder-tip pain, in the first trimester (could indicate ectopic pregnancy)
• Severe headache or vision changes: New, persistent headache, blurry vision, or seeing spots — could indicate preeclampsia, especially after 20 weeks
• Sudden swelling: Especially of the face, hands, or around the eyes, paired with headache or vision changes
• Fluid leaking: Gush or steady trickle of clear fluid before 37 weeks, or any fluid that's greenish, brownish, or bloody
• Reduced fetal movement: Fewer than 10 movements in 2 hours after 28 weeks once you've established your baby's normal pattern
• Regular painful contractions before 37 weeks: Five or more in an hour, especially with pelvic pressure or back pain
• Fever above 100.4°F (38°C): With or without other symptoms
• Persistent vomiting: Unable to keep fluids down for 24+ hours (can indicate hyperemesis)
• Persistent hopelessness or anxiety: Mental health support during pregnancy is safe, effective, and worth the call