Pregnancy Week-by-Week Guide

How Pregnancy Is Counted: Weeks, Trimesters, and Your Due Date

Pregnancy math confuses almost everyone at first, mostly because of one strange fact: your 40-week countdown starts before you're technically pregnant.

Why Week 1 Starts Before You're Actually Pregnant

Doctors date pregnancy from the first day of your last menstrual period (LMP), since that's usually the one date most people can pinpoint with confidence. Ovulation and conception typically happen about two weeks later, so by the time you're "4 weeks pregnant," you're really only about two weeks past conception. (ACOG) It's a strange system, but it's the one nearly every provider, app, and calculator uses, which is exactly why it's worth understanding early.

If your cycles are on the shorter or longer side, tracking your cycle for a month or two beforehand can make this starting date more accurate. Our guide on how to track your period accurately walks through exactly how to do that.

How the Three Trimesters Are Divided

The 40 weeks split into three trimesters of roughly 13 weeks each:

• First trimester: week 1 through 13 weeks, 6 days — fertilization and the formation of major organs
• Second trimester: 14 weeks through 27 weeks, 6 days — rapid growth and development
• Third trimester: 28 weeks through birth — weight gain and final organ maturity

That's according to the American College of Obstetricians and Gynecologists (ACOG), though your own provider's chart may round the boundaries slightly differently. Since pregnancy doesn't divide evenly into calendar months, here's roughly how weeks, trimesters, and "months" line up:

This is also why pregnancy gets described as "nine months" even though 40 weeks is closer to nine and a half — most calendar months run a little longer than four weeks.

How Your Due Date Is Estimated (and Why It's Not a Deadline)

Your estimated due date (EDD) is usually calculated by counting 280 days, or 40 weeks, forward from the first day of your last period. If your periods are irregular or you're unsure of that date, an early ultrasound measuring the embryo's crown-rump length can estimate your due date just as accurately, sometimes more so. (ACOG) If the two estimates differ by more than about a week, your provider will typically use the ultrasound date instead.

Here's the part worth remembering: only about 1 in 20 people actually deliver on their exact due date. (ACOG) Think of your due date as the center of a window rather than a deadline — most people deliver somewhere in the couple of weeks around it. Our pregnancy due date guide and what if your due date changes articles go deeper into both sides of this.

Pregnancy by Trimester: A Quick Overview

Before getting into individual weeks, it helps to zoom out. Each trimester has a distinct personality, and knowing what's coming next can make the harder stretches easier to plan around.

How Big Is Your Baby? A Week-by-Week Size Guide

Comparing your baby's size to a piece of fruit is a bit of a pregnancy tradition, and it turns out to be a genuinely useful way to picture growth that's otherwise hard to visualize.

How Doctors Measure Your Baby's Size

Early in pregnancy, your baby is curled up, so ultrasounds measure "crown-rump length" — from the top of the head to the bottom of the torso. Starting around week 20, your baby is measured head-to-heel instead, since the legs have uncurled enough to get a fuller length.

That measurement switch is exactly why size charts appear to show a sudden jump partway through the second trimester — your baby didn't grow ten centimeters overnight, the ruler just started measuring differently. Every figure below is an average, and plenty of healthy babies run smaller or larger than these numbers at any given week.

Week-by-Week Size Chart (Weeks 8–40)

*Week 21 shows the biggest single-week jump in the table because this is where measurement switches from crown-rump length to head-to-heel length, not because of one week of unusually fast growth.

Curious how your baby's estimated size compares to your own pregnancy weight gain so far? 100 Calculator's Pregnancy Weight Gain Calculator can help you see whether your gain is tracking within a typical range for your stage and starting weight.

Fetal Development Week by Week: Key Milestones

Growth in size is only half the story. Underneath that, your baby's organs, senses, and systems are developing on a fairly predictable schedule.

First Trimester Development (Weeks 1–13)

This is the busiest stretch for organ formation. The neural tube, which becomes the brain and spinal cord, forms within the first month, which is exactly why folic acid matters so early, often before a positive pregnancy test even shows up. By around week 6, the heart is beating, and by the end of the first trimester, arms, legs, fingers, toes, and external genitals have all started to form, along with early kidney and liver function.

Second Trimester Development (Weeks 14–27)

Development shifts from "building organs" to "growing and refining them." Your baby starts making coordinated movements you'll eventually feel, develops a sleep-wake pattern, grows hair, eyebrows, and eyelashes, and can often be seen sucking a thumb on ultrasound. By around 24 weeks, the lungs begin producing surfactant, a substance that will eventually let them expand with air after birth.

Third Trimester Development (Weeks 28–40)

The final stretch is largely about weight gain and fine- tuning. Your baby's brain and nervous system mature quickly, the lungs continue preparing for that first breath, and a layer of fat builds up to help with temperature regulation after birth. Most babies also settle into a head-down position sometime in the last several weeks, though there's variation in exactly when.

Some of the milestones parents ask about most:

• Heart begins beating: around week 6
• All major organs formed: by the end of week 13
• Fetal sex organs distinguishable on ultrasound: typically weeks 18–20
• Viability threshold: survival outside the womb becomes possible starting around week 24, with outcomes improving considerably each additional week
• Lungs reach maturity: generally by around week 36

Body Changes and Symptoms Through Pregnancy

While your baby is developing, your own body is going through some of the biggest hormonal shifts it will ever experience. Here's what tends to show up when.

First Trimester Symptoms

Nausea (often called morning sickness, though it can strike any time of day), fatigue, breast tenderness, and more frequent urination are the classic early signs. Mood swings are common too, driven by rapidly rising estrogen and progesterone.

Second Trimester Symptoms

Many people feel noticeably better here: nausea usually fades and energy comes back. New sensations take over instead, like round ligament pain (a quick, sharp twinge along the lower belly as the uterus stretches), a growing appetite, and the first flutters of fetal movement.

Third Trimester Symptoms

As your baby gets bigger, so does the physical load: Braxton Hicks contractions (irregular practice tightening of the uterus), lower back pain, swelling in the feet and ankles, shortness of breath, and trouble sleeping are all common in the home stretch.

Heartbeat, Movement, and Ultrasound Milestones

A few moments tend to stand out more than any others during pregnancy: hearing the heartbeat for the first time, feeling that first flutter of movement, and seeing your baby on a screen.

When Can You Hear Your Baby's Heartbeat?

Cardiac activity typically begins around week 5 to 6 and can often be seen on a vaginal ultrasound as early as week 6. A handheld Doppler at your provider's office usually picks it up reliably between weeks 10 and 12, though it's not unusual to wait until a follow-up visit if the first attempt comes up quiet — that's about timing and baby's position far more often than anything concerning.

When Will You Feel Your Baby Move? (Quickening)

That first sensation of movement is called quickening, and it's often described as flutters or tiny bubbles. If this is your first pregnancy, it typically shows up between weeks 18 and 25; if you've been pregnant before, you may recognize it a bit earlier, sometimes around week 16. (Cleveland Clinic) By around 28 weeks, providers generally suggest paying closer attention to movement patterns and mentioning any noticeable, ongoing decrease to your care team.

Common Ultrasound and Screening Timeline

Exact timing varies by provider and individual risk factors, but many pregnancies follow a similar general pattern of scans and screenings:

What to Avoid During Pregnancy

Most day-to-day life doesn't need to change dramatically during pregnancy, but a handful of foods, substances, and medications carry enough risk that health authorities recommend avoiding or limiting them.

Foods and Drinks to Limit or Avoid

• High-mercury fish, such as shark, swordfish, king mackerel, and tilefish
• Raw or undercooked meat, seafood, and eggs, which carry a higher risk of bacteria and parasites
• Deli meats and hot dogs, unless heated until steaming, due to listeria risk
• Unpasteurized dairy, juice, and soft cheeses, for the same reason
• Excess caffeine, generally kept under about 200 mg a day, roughly one 12-ounce cup of coffee

Lower-mercury fish like salmon, shrimp, and canned light tuna are actually encouraged in moderation, since they support your baby's brain development. (Mayo Clinic) 100 Calculator's Nutrition Calculator for Women can help you check whether your overall diet is meeting your needs during pregnancy.

Alcohol, Smoking, and Recreational Drugs

There's no known safe amount or safe time to drink alcohol during pregnancy, and no type of alcohol is considered an exception. (CDC) Smoking and vaping expose your baby to nicotine and carbon monoxide, raising the risk of low birth weight and preterm birth, and recreational drug use carries its own serious risks. If you're currently using any of these and finding it hard to stop, tell your provider — they can connect you with support rather than judgment.

Medications, Supplements, and Activities to Check With Your Doctor

The FDA recommends avoiding NSAIDs, such as ibuprofen, naproxen, and full-dose aspirin, from around 20 weeks of pregnancy onward, since they've been linked to low amniotic fluid and other complications later in pregnancy. (FDA) Acetaminophen is generally the go-to option for pain and fever, but it's still worth confirming with your provider or pharmacist before starting or continuing any medication or supplement, including ones that felt routine before pregnancy.

A few activities are also worth a second thought: hot tubs and saunas can raise your core body temperature more than is ideal, contact sports carry a higher fall or impact risk, and changing cat litter comes with a small risk of toxoplasmosis exposure. None of these are emergencies if they've already happened — they're simply easy things to adjust going forward.

Warning Signs That Need Immediate Medical Attention

Most pregnancy symptoms are uncomfortable rather than dangerous, but a smaller set of signs should never be brushed off, according to the CDC's guidance on urgent maternal warning signs. (CDC) Contact your provider right away, or go to an emergency room, if you notice:

• Heavy vaginal bleeding
• A severe headache that won't go away, especially with vision changes
• Sudden or severe swelling in the face or hands
• Severe or constant abdominal pain that doesn't ease up
• A fever of 100.4°F (38°C) or higher
• Sudden shortness of breath or chest pain
• A noticeable decrease in your baby's movement after you've become familiar with their usual pattern
• Regular contractions, pelvic pressure, or fluid leakage before 37 weeks

None of these guarantee something is seriously wrong, but they're exactly the kind of symptoms your care team would rather hear about early. If something feels different or concerning and you're not sure whether it counts, it's always reasonable to call and ask.

Common Pregnancy Myths vs. Facts

Pregnancy attracts more folklore than almost any other life stage. Here's the reality behind a few of the most common claims.

Building Your Prenatal Care Schedule

Regular prenatal visits are how your provider tracks your baby's growth and catches potential issues early, so it's worth understanding roughly what the schedule looks like.

A Typical Visit Schedule

The traditional model, in place for decades, calls for a visit about once a month until 28 weeks, every two weeks until 36 weeks, and weekly from 36 weeks until delivery. More recently, ACOG has also endorsed "tailored" prenatal care models that combine fewer in-person visits with telehealth check-ins for average-risk pregnancies, so your own provider's schedule may look somewhat different. (ACOG) Either way, the goal is the same: regular monitoring of your blood pressure, weight, and your baby's growth throughout pregnancy.

Common Tests and Screenings by Trimester

Your first prenatal visit, ideally before 10 weeks, typically covers your medical history, an initial round of bloodwork, and confirmation of your due date. From there, the screening timeline in the section above (NT scan, anatomy scan, glucose screening, and GBS testing) fills in most of the remaining major checkpoints, with blood pressure and fundal height checked at almost every visit from the second trimester onward.

If irregular cycles made it tricky to pin down your original due date, our guide on signs of an irregular menstrual cycle explains what can throw off that first estimate.

Tips for Tracking Your Pregnancy Week by Week

A little structure makes the next several months easier to follow, whether you're a spreadsheet person or prefer to keep things simple.

1. Confirm your due date early using your last period and, if needed, an early ultrasound, so every other estimate in this guide lines up with your actual week.
2. Log symptoms as they happen, not just dates. A quick note about nausea, energy, or movement patterns is genuinely useful at your next appointment.
3. Take a weekly bump photo if you enjoy it, rather than comparing yourself to other people's photos online. Every body carries pregnancy differently, and your own week-by-week series will mean more to you than anyone else's.
4. Write down questions between visits so you're not trying to remember them on the spot during a short appointment window.
5. Involve a partner or support person in tracking milestones like the anatomy scan or first felt movement — it helps the pregnancy feel shared rather than solitary.

Once you know your due date, 100 Calculator's Pregnancy Due Date Calculator can show your current week at a glance, and the Baby Growth Memory Calendar is a simple way to keep a running record once your baby arrives.

More Pregnancy and Fertility Guides

If you're planning a pregnancy, currently expecting, or just trying to understand your cycle better, these related guides dig deeper into the topics touched on above.

Frequently Asked Questions