Your First Trimester Journey: Week by Week

Welcome to your in-depth guide on what to expect during your pregnancy journey! We’re going over each pregnancy trimester so you know what to expect, including key symptoms, changes, your baby’s development, and what to look out for. Our complete week-by-week pregnancy  guide will cover:

• First trimester (conception to 12 weeks)
• Second trimester (13 to 27 weeks)
• Third trimester (28-40 weeks)
• Fourth trimester (Postpartum)

The first trimester is critical for pregnancies, and your baby’s development will depend heavily on your health and wellness. The stages of pregnancy won’t look the same for everybody, but we hope to cover important milestones and provide nutrition tips. Let’s dive into your first trimester!

What Is the First Trimester?

The first trimester refers to the period from conception to 12 weeks, starting from the first day of your last period and ending at the end of the third month of pregnancy. While you may have an inkling you’re pregnant, on average, nearly 60% of women do not experience the typical pregnancy symptoms (nausea, fatigue, etc.) until the fifth or sixth week of pregnancy. Additionally, rapidly fluctuating hormones can cause mood swings.

Clinicians describe the first trimester as the “most critical” period for your baby's early development, so it's a great time to implement healthy lifestyle practices to support these processes.

First Trimester Pregnancy Symptoms

While everyone’s experience is different, during the first trimester, you may experience the following early pregnancy symptoms:

• Nausea and vomiting
• Fatigue
• Breast tenderness and soreness
• Food aversions
• Mood swings
• Bloating and constipation
• Food cravings
• A metallic taste in the mouth

Spotting or bleeding is a somewhat regular first-trimester symptom, affecting anywhere between 15% and 25% of women. However, you should still contact your healthcare provider if you experience either.

Week-by-Week Breakdown of the First Trimester

Here are the key fetal development milestones, common symptoms, and changes you can expect in the first 12 weeks of pregnancy.

Weeks 1–4: Conception & Implantation

Key Fetal Developments (fertilization, implantation)

A lot happens during these first four weeks:

• During the first 2 weeks of pregnancy, you aren’t pregnant, since the “clock” starts on the first day of your last period, so you’re ovulating and preparing for pregnancy
• During ovulation, your ovary releases an egg, and if you’ve had unprotected penetrative sex within your fertility window, then the egg may be fertilized by sperm. 
• Within a few days, the fertilized egg implants into the lining of the womb, and now you’re pregnant.

Common Early Pregnancy Symptoms

For many women, the first sign of pregnancy is a missed period, and you may even start to experience early pregnancy signs like nausea or fatigue. 

Nutrition Tips for the First Four Weeks

If you’re actively trying to conceive, you’re probably already paying attention to your nutrition and taking steps such as balancing your hormones, optimizing your sleep schedule, or even taking a prenatal vitamin (yes, you can take them before you’re pregnant!). Consider the following:

• Eat a whole foods diet with lots of colorful veggies, fruits, lean protein, fiber, and plenty of water.
• Add a prenatal vitamin to your routine, as it can be challenging to meet your daily nutritional goals, especially during pregnancy, when your nutritional needs increase. Nutrients like folate, choline, and vitamin B12 support key processes in early pregnancy, like the early development of the fetal brain and spinal cord, including neural tube formation.
• Stop smoking and drinking alcohol (ideally, when you are actively trying to conceive)
  

Weeks 5-8: Early Organ Development

Major Milestones

Your baby grows more in the first 12 weeks than at any other stage of your pregnancy. During weeks 4-8:

• The baby grows from the size of a poppy seed to the size of a raspberry
• The embryo splits into three germ layers, which will grow into different parts of your baby’s body:
  • Ectoderm: The outer layer, which will develop into the brain and nervous system, eye lenses, tooth enamel, skin and nails
  • Mesoderm: The middle layer of the embryo, which will develop into bones, the heart, muscles and blood vessels
  • Endoderm: The innermost layer of the embryo, which will develop into the breathing (lungs) and digestive (gut, stomach and bladder) systems
• Your body is creating the placenta, which will deliver nutrients and oxygen to your baby
• Your baby’s head, heart, and limbs are starting to develop.

First Trimester Symptoms Make an Appearance

During these weeks, the majority of women start experiencing pregnancy symptoms such as morning sickness, breast tenderness or soreness, fatigue, strange food cravings, frequent urination, and bloating or constipation. You may also experience occasional anxiety or stress.

Increasing Nutritional Needs

Blood volume increases by about 50% during pregnancy to provide nutrients and oxygen to the uterus, among other physical demands. Adjust your diet to incorporate nutrients, and it’s never too early to drink more water. Look for a prenatal vitamin with at least 400mcg of folate.

Weeks 9–12: Baby’s Features Form

Significant Developments

Your baby’s features start to take shape–The mouth, tongue, nose and ears rapidly develop. The baby’s eyes even have some color and can react to light; they even have fingernails by the end of the 12th week. Additionally:

• There are little ridges on the hands and feet that will eventually become fingers and toes
• The baby’s muscles are formed enough that they can start moving in their amniotic sac
• Baby teeth are budding in the baby’s gums

By the end of the 12th week, your baby is the size of a plum, and it's not uncommon to still have your pre-pregnancy stomach. 

Physical and Emotional Changes Persist

Morning sickness can start to abate as early as the 10th or 12th week, though for many women, it persists until about the 16th week of pregnancy. As you near the 2nd trimester, you may experience:

• A reduction in early pregnancy symptoms like fatigue, nausea, and food aversions
• Larger breasts and thicker, shinier hair, and changes in the skin (more oily, possibly with acne)
• Higher levels of hormones, including human chorionic gonadotropin (hCG), the “pregnancy hormone”, and estrogen and progesterone, all of which help increase blood supply to your uterus.
• The appearance of a skin condition called chloasma, melasma, or “mask of pregnancy”, causing darker hyperpigmentation primarily on the face

Preparing for the Second Trimester

This is the final stretch of the first trimester. As you prepare to enter the 2nd trimester, ensure you’re still taking your daily prenatal vitamins, staying active, and drinking enough water!

First Trimester Baby Development Overview

Your baby grows from a little pea-sized shape to an almost fully-formed human with arms, legs, and facial features during the first trimester, growing from an embryo to a fetus. Fetal development during the first trimester includes:

• Formation of the brain, spinal cord, and heart: During the first 12 weeks, the foundations of your baby’s most important organ systems, including the central nervous and digestive systems, will form. 
• Development of major organs like the liver and kidneys: The embryo splits into three layers, the ectoderm, mesoderm and endoderm, which produce your baby’s tissues, organs, and organ systems.
• Facial features, limbs, fingers, and toes begin to form: By the 12th week, every facial feature has taken shape, so your baby will have a mouth, lips, nose, ears, and eyes.

Essential First Trimester Health & Nutrition Tips

It’s essential to look after your health in the first trimester to support both your well-being and your baby’s development. Focus on nutrition, managing stress, and ensuring you’re getting enough rest. Also, unless otherwise instructed by your healthcare provider, you can continue to exercise.

First Trimester Nutrition

You may be exhausted and find it hard to keep food down. However, this is a critical period to continue incorporating green leafy greens, whole grains, lean proteins, healthy fats, and a high-quality prenatal vitamin. Pregnancy is physically and nutritionally demanding, so these are key steps to help avoid nutritional depletion.

Foods to Avoid in the First Trimester

Aside from stopping smoking and drinking alcohol, you should also avoid the following during pregnancy:

• Raw fish, such as sushi
• Raw or undercooked meat and eggs
• Processed meat, like deli meats
• Raw sprouts (alfalfa, broccoli, lentil, etc.)

First Trimester Medical Appointments & Tests

Prenatal care is critical for a healthy pregnancy. Your healthcare provider will monitor your baby’s development and your health, and catching any potential issues early can significantly improve the outcome.

What Happens at the First Prenatal Visit?

You will most likely have your first prenatal visit between weeks six and nine. During this appointment, your provider will:

• Confirm the pregnancy with a pregnancy test (even if you took one at home, the doctor still needs to confirm for their records)
• Review your personal and family medical history
• Conduct a physical exam that includes blood pressure and a breast and pelvic exam
• Confirm your due date
• Detail any necessary screenings or tests, perform an ultrasound, and collect blood or tissue samples. 

Common First Trimester Screenings

There are several “standard” first-trimester screenings and tests your provider will schedule for you:

• Blood Tests: Your blood type and Rh factor, red blood cell count, a sexually transmitted infection (STI), and a rubella antibody test
• Ultrasound: Within the first 1-2 prenatal appointments, your provider will conduct an ultrasound to check for the baby’s heartbeat and position in your uterus
• Chorionic Villus Sampling (CVS): Ob/Gyns usually perform the test between the 10th and 12th weeks of pregnancy. It tests for chromosomal abnormalities, and some women may choose to undergo this screening if they have a family history of abnormalities or previously had an abnormal ultrasound.

Many providers now offer optional genetic testing, such as Noninvasive prenatal testing (NIPT), to determine the risk of certain genetic abnormalities.

Emotional & Mental Health in the First Trimester

It is normal to experience heightened emotions during the first trimester. In fact, due to fluctuating hormones, many women experience first-trimester mood swings. It’s also common to experience occasional anxiety and stress as you adjust to being pregnant and think about how things will have to change once the baby is born.

First Trimester FAQs

How long is the first trimester?

The first trimester lasts 12 weeks, from the first day of your last period to the end of the 3rd month of pregnancy.

What are common first-trimester symptoms?

Typical first trimester symptoms include:

• Nausea and vomiting
• Fatigue
• Breast tenderness and soreness
• Food aversions
• Mood swings
• Bloating and constipation
• Food cravings
• A metallic taste in the mouth

What should I avoid in the first trimester?

During the first trimester, you should avoid raw fish, fish high in mercury (swordfish, mackerel, crawfish, etc), raw or undercooked meats and eggs, and processed meat.

Is it normal to feel anxious during the first trimester?

Yes, many women report an increase in feelings of occasional anxiety and stress during pregnancy, including the first trimester.

When will morning sickness stop?

For many women, morning sickness persists through the first trimester, easing up between the 12th and 16th weeks. Some helpful tips that may help manage morning sickness are taking a magnesium or vitamin B6 supplement, keeping ginger on hand, and having smaller meals throughout the day.