First Trimester: Week by Week Guide to Early Pregnancy

You just found out you’re pregnant — or maybe you’re only a few weeks in and trying to figure out what’s happening inside your body. Either way, the first trimester can feel like a whirlwind of excitement, anxiety, and a lot of Googling at 2 AM. You’re not alone, and everything you’re feeling is completely normal.

The first trimester spans from week 1 through week 13 of pregnancy. During these early weeks, your baby goes from a microscopic cluster of cells to a fully formed (though very tiny) human being with a beating heart, developing organs, and even tiny fingers and toes. Meanwhile, your body is working overtime behind the scenes — even before you start showing.

Let’s walk through it week by week so you know exactly what to expect.

How Pregnancy Weeks Are Counted

Before we dive in, a quick note that surprises many first-time parents: pregnancy is dated from the first day of your last menstrual period (LMP), not from the day of conception. This means that during “weeks 1 and 2,” you aren’t actually pregnant yet — your body is preparing to ovulate and conceive.

Actual fertilization typically happens around week 2 or 3 of your cycle. So when your doctor says you’re “6 weeks pregnant,” your baby has really only been developing for about 4 weeks. It’s a bit confusing, but it’s the standard used worldwide.

Weeks 1–4: Conception and Implantation

What’s happening with baby: During weeks 1 and 2, your body is going through its normal menstrual cycle. Ovulation occurs, and if an egg meets a sperm, fertilization takes place — usually in the fallopian tube. The fertilized egg (now called a zygote) begins dividing rapidly as it travels toward your uterus.

By the end of week 3, the zygote has become a blastocyst — a tiny ball of about 200 to 300 cells. Around days 6 to 10 after fertilization, the blastocyst burrows into the uterine lining in a process called implantation.

By week 4, the embryo is about the size of a poppy seed. The placenta is beginning to form, and the cells are already organizing into layers that will become different organ systems.

What you might feel:

Most people feel nothing at all during these weeks
Some experience light spotting (implantation bleeding) around week 4
Mild cramping similar to period cramps is common
A missed period is usually the first real clue

Key milestone: A home pregnancy test can usually detect the hormone hCG (human chorionic gonadotropin) around the time of your missed period — roughly 4 weeks after your LMP.

Weeks 5–6: The Heartbeat Begins

What’s happening with baby: Week 5 is a major turning point. The neural tube — which will become the brain and spinal cord — is forming. The embryo is only about 2 millimeters long, roughly the size of a sesame seed, but development is incredibly rapid.

By week 6, the embryo’s heart begins to beat. It starts as a simple tube-like structure that folds and divides into chambers, beating at roughly 100 to 120 beats per minute. Arm and leg buds are starting to appear, and the very earliest facial features — tiny pits where the eyes will be, small openings for the mouth and nostrils — are taking shape.

What you might feel:

Morning sickness may begin (despite the name, it can strike at any time of day)
Breast tenderness and swelling
Fatigue — you may feel more exhausted than you’ve ever been
Frequent urination as your uterus starts to expand and blood volume increases
Mood swings driven by rising hormone levels

What to do: If you haven’t already, schedule your first prenatal appointment. The American College of Obstetricians and Gynecologists (ACOG) recommends that prenatal care begin as soon as you know you’re pregnant. Make sure you’re taking a prenatal vitamin with at least 400 micrograms of folic acid daily — this is critical for neural tube development.

Weeks 7–8: Rapid Growth

What’s happening with baby: At week 7, your baby is about the size of a blueberry (roughly 10 mm). The brain is growing rapidly, and limb buds are elongating into more defined arms and legs. Fingers and toes haven’t separated yet — the hands and feet look more like tiny paddles.

By week 8, the embryo is now about 16 millimeters (about the size of a raspberry). All major organs have begun forming. The intestines are developing, the liver is producing red blood cells, and the skeleton is starting to take shape — though it’s still made of soft cartilage, not bone. Elbows are becoming visible, and the toes and fingers are starting to separate.

What you might feel:

Morning sickness may intensify — this often peaks between weeks 8 and 10
Food aversions and cravings may appear
Your sense of smell may be heightened
Bloating and constipation due to progesterone slowing digestion
You might notice your waistband is slightly tighter, though a visible bump is still weeks away

Weeks 9–10: From Embryo to Fetus

What’s happening with baby: A big naming milestone happens at week 9: your baby officially transitions from “embryo” to “fetus.” At this point, all essential organs have formed, and the remainder of pregnancy will focus on growth and maturation.

The fetus is roughly 23 millimeters — about the size of a grape. Tiny muscles are forming, and your baby can make small movements, though you won’t feel them yet. The tail-like structure from earlier weeks has disappeared, and the baby is looking more human.

By week 10, bones are beginning to harden. Fingernails are starting to form. The kidneys are producing urine. The baby’s face is becoming more recognizable, with a defined upper lip and two small nostrils.

What you might feel:

Visible veins on your chest and breasts as blood volume increases (by the end of pregnancy, your blood volume will increase by about 45%)
Round ligament discomfort — occasional sharp or dull aches in your lower abdomen
Continued fatigue and nausea (hang in there — relief is often just a few weeks away)

Weeks 11–12: Finishing the First Trimester

What’s happening with baby: At week 11, the fetus is about 4 centimeters long and weighs around 7 grams — roughly the size of a fig. The head is still disproportionately large (about half the body length), which is normal. Hair follicles are forming, and the genitals are beginning to differentiate, though it’s usually too early to determine sex on ultrasound.

By week 12, the fetus measures about 5.5 centimeters. Reflexes are developing — your baby may start to curl their toes or make sucking movements. The digestive system is practicing contractions that will eventually process food. The bone marrow is beginning to produce white blood cells, a critical part of the immune system.

What you might feel:

Many people notice nausea starting to ease around week 12
Your energy levels may begin to improve
The “pregnancy glow” — increased blood flow and oil production can make your skin look more radiant (though some people experience acne instead)
Your uterus has grown to roughly the size of a grapefruit and is rising above your pubic bone

Week 13: Welcome to the Second Trimester

What’s happening with baby: At week 13, the fetus is about 7.5 centimeters long and weighs roughly 23 grams. Fingerprints are forming — your baby already has a unique identity. The vocal cords are developing. If you’re having a girl, her ovaries already contain about 2 million eggs.

What you might feel:

A noticeable boost in energy for many people
Morning sickness typically fades (though it persists longer for some — that’s also normal)
You may start showing, especially with a second or later pregnancy
Food may start tasting good again!

Prenatal Appointments and Tests in the First Trimester

Your first prenatal visit usually happens between weeks 8 and 10. Here’s what to expect:

Complete health history — your doctor will ask about your medical background, medications, and family history
Blood tests — to check your blood type, Rh factor, hemoglobin levels, immunity to rubella and chickenpox, and screening for HIV, syphilis, and hepatitis B
Urine test — to check for urinary tract infections and establish a baseline for protein and glucose levels
Blood pressure measurement — establishing your baseline is important for monitoring preeclampsia risk later
First ultrasound — usually between weeks 8 and 12. This confirms the pregnancy, estimates due date, checks for a heartbeat, and determines if you’re carrying multiples

Between weeks 11 and 14, you may be offered first-trimester screening, which combines a blood test with a nuchal translucency ultrasound (measuring the fluid at the back of the baby’s neck) to assess the risk of certain chromosomal conditions like Down syndrome. Non-invasive prenatal testing (NIPT) — a blood test that analyzes fetal DNA fragments in your blood — may also be offered and can be done as early as week 10.

Tips for a Healthy First Trimester

Start prenatal vitamins — 400 micrograms of folic acid daily (600 mcg total is the ACOG recommendation during pregnancy). Folic acid significantly reduces the risk of neural tube defects.
Stay hydrated — aim for 8 to 12 cups of water daily. This helps with fatigue, constipation, and supports the dramatic increase in blood volume.
Eat what you can — if nausea is an issue, don’t stress about a perfect diet. Small, frequent meals with bland foods can help. For detailed guidance, check our prenatal nutrition guide.
Get enough rest — first-trimester fatigue is real and intense. Listen to your body. Nap when you can.
Avoid harmful substances — no alcohol, tobacco, or recreational drugs during pregnancy. Limit caffeine to under 200 milligrams per day (roughly one 12-ounce cup of coffee), per ACOG guidelines.
Know which medications are safe — check with your doctor before taking any medication, including over-the-counter remedies and herbal supplements. Acetaminophen (paracetamol) is generally considered safe, but ibuprofen and aspirin should be avoided unless specifically prescribed.
Move your body — unless your doctor says otherwise, gentle exercise is safe and beneficial. Walking, swimming, and prenatal yoga are great options. See our exercise guide for trimester-specific recommendations.

When to Call Your Doctor

While many first-trimester symptoms are normal, certain signs warrant immediate medical attention:

Heavy bleeding (soaking a pad in an hour) or passing clots
Severe abdominal or pelvic pain that doesn’t go away
Severe vomiting that prevents you from keeping any food or liquid down for more than 24 hours (this could be hyperemesis gravidarum — read more in our morning sickness guide)
Fever above 38°C (100.4°F)
Pain or burning during urination (could indicate a UTI, which can be harmful if untreated)
Dizziness or fainting

Trust your instincts. If something feels wrong, it’s always better to call and be reassured than to wait and worry.

Frequently Asked Questions

Is it normal to have cramping in early pregnancy?

Yes, mild cramping is very common in early pregnancy. It’s usually caused by the uterus stretching and the embryo implanting in the uterine lining. However, if cramping is severe, one-sided, or accompanied by heavy bleeding, contact your doctor immediately, as it could indicate an ectopic pregnancy or miscarriage.

When will I start showing?

This varies widely. Many first-time mothers don’t have a noticeable bump until 12 to 16 weeks. With subsequent pregnancies, you may show earlier because the abdominal muscles have already stretched. Body type, weight, and the position of your uterus also play a role.

Can I fly during the first trimester?

Generally, yes. Air travel is considered safe during uncomplicated pregnancies. ACOG notes that the safest time to fly is during the second trimester (weeks 14–28), but first-trimester flying is fine for most people. Stay hydrated, move around the cabin regularly, and wear compression socks for flights longer than 4 hours.

How much weight should I gain in the first trimester?

For people with a normal pre-pregnancy BMI (18.5–24.9), ACOG recommends gaining about 1 to 2 kilograms (2 to 4 pounds) in the entire first trimester. Don’t worry if morning sickness causes you to lose weight early on — you’ll have time to catch up in the second and third trimesters.

Is it safe to have sex during the first trimester?

Yes, for most people, sex during pregnancy is perfectly safe. The baby is well-protected by the amniotic sac and the strong muscles of the uterus. However, your doctor may recommend abstaining in certain situations, such as if you have a history of miscarriage, unexplained bleeding, or a low-lying placenta.

Sources

American College of Obstetricians and Gynecologists (ACOG). “Prenatal Development: How Your Baby Grows During Pregnancy.” FAQ156, 2021.
ACOG. “Prepregnancy Counseling.” Committee Opinion No. 762, 2019.
ACOG. “Nutrition During Pregnancy.” FAQ001, 2022.
World Health Organization (WHO). “WHO Recommendations on Antenatal Care for a Positive Pregnancy Experience.” 2016.
NHS. “You and Your Baby at 0–8 Weeks Pregnant.” 2024.
NHS. “You and Your Baby at 9–12 Weeks Pregnant.” 2024.
Moore KL, Persaud TVN, Torchia MG. “The Developing Human: Clinically Oriented Embryology.” 11th ed. Elsevier, 2019.