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What to Expect, Baby Development

Written by Stephanie Watson

Reviewed by Traci C. Johnson, MD on August 12, 2022

In this Article

• What Is the First Trimester of Pregnancy?
• First Trimester Changes in Your Body
• Baby’s Growth in the First Trimester
• First Trimester To-Dos
• Emergency Symptoms During the First Trimester

What Is the First Trimester of Pregnancy?

The first trimester is the earliest phase of pregnancy. It starts on the first day of your last period -- before you’re even actually pregnant -- and lasts until the end of the 13th week. It’s a time of great anticipation and of rapid changes for both you and your baby. Knowing what to expect will help you get ready for the months ahead.

First Trimester Changes in Your Body

Pregnancy is different for every woman. Some women glow with good health during those first 3 months; others feel absolutely miserable. Here are some of the changes you might notice, what they mean, and which signs warrant a call to your doctor.

Bleeding. About 25% of pregnant women have slight bleeding during their first trimester. Early in the pregnancy, light spotting may be a sign that the fertilized embryo has implanted in your uterus. But if you have severe bleeding, cramping, or sharp pain in your belly, call the doctor. These could be signs of a miscarriage or ectopic pregnancy (a pregnancy in which the embryo implants outside of the uterus).

Breast tenderness.Sore breasts are one of the earliest signs of pregnancy. They're triggered by hormonal changes, which are getting your milk ducts ready to feed your baby. Your breasts will probably be sore throughout the first trimester. Going up a bra size (or more) and wearing a support bra can make you feel more comfortable. You probably won’t go back to your regular bra size until after your baby is finished nursing.

Constipation. During pregnancy, high levels of the hormone progesterone slow down the muscle contractions that normally move food through your system. Add to that the extra iron you're getting from your prenatal vitamin, and the result is uncomfortable constipation and gas that can keep you feeling bloated throughout your pregnancy. Eat more fiber and drink extra fluids to keep things moving more smoothly. Physical activity can also help.

If your constipation is really bothering you, talk to your doctor about what mild laxative or stool softeners are safe to use during pregnancy.

Discharge. It's normal to see a thin, milky white discharge (called leukorrhea) early in your pregnancy. You can wear a panty liner if it makes you feel more comfortable, but don't use a tampon because it could put germs into your vagina. If the discharge smells really bad, if it’s green or yellow, or if there's a lot of clear discharge, call the doctor.

Fatigue. Your body is working hard to support a growing baby. That means you’ll get tired more easily than usual. Take naps or rest when you need to during the day. Make sure you're getting enough iron. Too little can lead to anemia, which can make you even more tired.

Food likes and dislikes. Although you may not want a bowl of mint chip ice cream topped with dill pickles, as the old stereotype goes, your tastes can change while you're pregnant. More than 60% of pregnant women have food cravings. More than half have foods they really don’t like. Giving in to cravings from time to time is OK, so long as you eat healthy, low-calorie foods most of the time. The exception is pica -- a craving for nonfoods like clay, dirt, and laundry starch, which can be dangerous for you and your baby. If you experience this kind of craving, report it to your doctor right away.

Peeing a lot. Your baby is still pretty small, but your uterus is growing and it's putting pressure on your bladder. As a result, you may feel like you have to go to the bathroom all the time. Don't stop drinking fluids -- your body needs them -- but do cut down on caffeine (which stimulates your bladder), especially before bedtime. When nature calls, answer it as soon as you can. Don't hold it in.

Heartburn. During pregnancy, your body produces more of the hormone progesterone. It relaxes smooth muscles, like the ring of muscle in your lower esophagus, the tube that connects your mouth and stomach. These muscles normally keep food and acids down in your stomach. When they loosen up, you can get acid reflux, otherwise known as heartburn. To avoid the burn:

• Eat a few small meals throughout the day.

• Don't lie down right after you eat.

• Avoid greasy, spicy, and acidic foods (like citrus fruits).

• Try raising your pillows when you sleep.

Mood swings. Increased fatigue and changing hormones can put you on an emotional roller coaster that takes you from joyous to miserable, or from hopeful to terrified in a matter of seconds. It's OK to cry, but if you feel overwhelmed, try to find an understanding ear. You can talk to your partner, a friend, a family member, or even a professional.

Morning sickness.Nausea is one of the most common pregnancy symptoms. Up to 85% of pregnant women have it. It results from hormone changes in your body and it can last through the entire first trimester. For some pregnant women, nausea is mild. Others can't start their day without vomiting. Nausea is usually worse in the morning (hence the name, "morning sickness"). To calm your nausea, try eating small, bland, or high-protein snacks (crackers, meat, or cheese) and sipping water, clear fruit juice (apple juice), or ginger ale. You may want to even do this before you get out of bed. Avoid any foods that make you sick to your stomach. Nausea itself isn't anything to worry about, but if it’s severe or just won’t go away, it can affect the amount of nutrition your baby gets. Call your doctor if you can't stop throwing up or can't keep down any food.

Weight gain. Pregnancy is one of the few times in a woman's life when weight gain is considered a good thing, but don't overdo it. During the first trimester, you should gain about 3-6 pounds (your doctor may suggest you adjust your weight gain up or down if you started your pregnancy underweight or overweight). Although you're carrying an extra person, you really aren’t eating for two. You only need about an extra 150 calories a day during the first trimester. Get those calories the healthy way, by adding extra fruits and vegetables, milk, whole-grain bread, and lean meat to your diet.

Baby’s Growth in the First Trimester

During the first 13 weeks, your baby changes from a fertilized egg into a fully-formed fetus. All the major organs and systems are taking shape. That means your baby could be harmed if you use street drugs, have an illness, or get exposed to radiation. Here’s what’s happening:

• The fertilized egg becomes a cluster of rapidly dividing cells that implants in your uterus. The placenta, umbilical cord and amniotic sac all start to grow.

• Your baby’s nervous system changes from an open neural tube to a brain and spinal cord. Nerves and muscles start working together. Your baby can move on its own, but it’s too soon for you to feel it.

• The heart takes shape and begins to beat. You can hear it on ultrasound as early as week 6. It beats 120 to 160 times per minute. Red blood cells are forming.

• Your baby develops a digestive system, including intestines and kidneys.

• They have lungs and other major organs, but they’re not fully developed.

• A soft skeleton is starting to grow.

• Your baby starts to look like a baby, with arms, legs, fingers, and toes. Their face gets eyes, ears, a nose, and mouth. A tongue and tooth buds grow. Eyelids cover your baby’s eyes, and by the end of the trimester, they even have fingernails.

• Genitals start to grow, but it’s too early to tell by ultrasound whether you’re having a girl or a boy.

By the end of the first trimester, your baby will be about 2 ½-3 inches long.

First Trimester To-Dos

Having a baby is one of the most joyous times in many women's lives. From thinking about the day you'll bring your little one home, to picking a name and nursery colors, the excitement is intense. But you also need to take some practical steps during the first trimester, including:

• Choose a doctor. Do you want an obstetrician or a midwife? Get referrals and find out what your health insurance covers.

• Schedule a prenatal visit as soon as you know you’re pregnant. You’ll cover a lot of ground in the first appointment. The doctor will take a full medical history and talk to you about your lifestyle and health habits. They’ll figure out your due date. You’ll also have blood and urine tests and possibly a pelvic exam.

• Continue with prenatal visits every 4 weeks. The doctor will check your weight and blood pressure, test your urine and listen to your baby’s heartbeat.

• Learn what other tests and screenings you may need, such as tests to look for genetic problems with your baby.

• Start taking a prenatal vitamin with at least 400 micrograms of folic acid to help your baby’s brain and spinal cord grow properly.

• Ask your doctor what prescription and over-the-counter medicines you can still safely take.

• Take a look at your diet and make any changes you need to make sure you and your baby get the right nutrition. Drink plenty of water.

• Break bad habits like smoking and illegal drug use. Cut out alcohol and cut down on caffeine.

• Keep up your workout routine, but listen to your body. You may need to change what kind of exercise you do, or ease up a little.

• Research what it costs to have a baby and start making changes. Will you have to pay for child care? Will you cut back on work? Draw up a new budget that reflects the new addition.

• Decide when and how you’ll share your news. You may want to wait until you’ve heard the baby’s heartbeat, or made it safely past the first trimester. It’s also smart to find out your company’s policies on maternity leave and learn your rights before you tell your boss.

Emergency Symptoms During the First Trimester

Any of these symptoms could be a sign that something is seriously wrong with your pregnancy. Don't wait for your prenatal visit to talk about it. Call your doctor right away if you experience:

• Severe abdominal pain
• Heavy bleeding
• Severe dizziness
• Rapid weight gain or too little weight gain

Health & Pregnancy Guide

1. Getting Pregnant
2. First Trimester
3. Second Trimester
4. Third Trimester
5. Labor and Delivery
6. Pregnancy Complications
7. All Guide Topics

1st trimester of pregnancy: what happens to the fetus

1st trimester: 1st-12th weeks

The gestational age is calculated from the first day of the last menstruation, since it is difficult to determine the exact day of conception. Since conception usually occurs in the middle of the menstrual cycle, you are not actually pregnant during the first two weeks, but this period is counted as the beginning of pregnancy.

As soon as the fertilization of the egg takes place around the 3rd week, the hormones begin to produce changes in your body little by little. As a result, you may experience some of the following symptoms:

• Morning sickness. As a result of rising levels of hormones characteristic of pregnancy, up to 80% of women in the 1st trimester experience morning sickness with symptoms such as nausea and vomiting. The idea that such malaise is observed only in the morning is a common misconception. In fact, symptoms can appear at any time of the day or night. Up to 1 in 5 women experience morning sickness in the 2nd trimester of pregnancy and can sometimes persist throughout pregnancy.

  If you experience morning sickness, avoid foods that make you sick, eat little and often, avoid fatty and spicy foods, drink more water. If you experience severe symptoms or symptoms that bother you, see your doctor.

• Breast changes. The mammary glands will begin to increase in size, soreness may appear. The nipples will increase in size, become darker and more protruding.
• Fatigue. High levels of the hormone progesterone can make you feel tired and sleepy. Rest as often as possible in a horizontal position with your legs up and eat as well as possible, which is not easy if you are experiencing morning sickness!
• Increased emotionality. A higher level of emotionality, manifested as a result of an increase in hormone levels, is normal. Understanding and patience on the part of your partner and loved ones is very important here.
• Food likes and dislikes. You may find yourself intolerant of one food and addicted to another. This is usually not a problem, unless you feel like eating weird foods like chalk. If you are concerned about the situation, contact your doctor.
• Frequent urination. As your fluid levels increase and your uterus puts pressure on your bladder, you will become more likely to visit the toilet. Go to the toilet as soon as you feel the need - this minimizes the pressure on the bladder.
• Feeling of dizziness. Sometimes you may feel a little dizzy (this is due to hormonal changes). Try not to stay on your feet for a long time and slowly rise from a sitting or lying position. If you experience severe dizziness, contact your doctor immediately.
• Heartburn and constipation. Your digestive system will slow down to give you more time to digest your food. This can lead to heartburn and constipation. To help manage heartburn, try to eat small meals at regular intervals and avoid fried or spicy foods and carbonated drinks. Constipation is helped by eating a diet rich in fiber, maintaining physical activity and drinking plenty of water.

• Approximately 7 days after fertilization, the embryo implants in the uterine wall. The placenta, umbilical cord and amniotic sac will begin to form to provide nourishment and protection to the embryo.
• By the end of the first 12 weeks of pregnancy, the uterus is palpable through the wall of the abdomen, the abdomen will begin to grow.

By the end of the 1st trimester:

• All the main organs of the baby are formed, the circulatory system works.
• The development of the sexual organs has begun.
• Fingers are formed on the hands and feet, nails have appeared.
• Facial features have formed.
• The length of the baby's body is about 6 cm from the head to the lower part of the body, he is already recognizable. The baby moves in the amniotic sac, but you don't feel it move yet.

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First trimester: little life | Home

Table of contents:

• When the baby's heart beats and what he will look like
• How does a baby eat and where does his energy come from
• Is it true that the baby is already moving
• Drinking and pissing

When your baby's heart beats and who he will look like

Two weeks before conception (obstetric weeks are counted from the first day of your last period, and nominally these are the first weeks of your pregnancy) your body has prepared for this event: in ovarian follicle matured and then ovulation occurred - the release of the egg from the ruptured ovarian follicle ¹ .

Then an incredible event happens - life is born. One of the hundreds of millions of sperm finds your egg. And almost immediately after that, your baby has a unique set of chromosomes. It contains information about the child's gender, eye color, hair color, and even character traits. And this despite the fact that it is still quite tiny - only 0.1 millimeters, like the tip of a hair. In the near future, he will bear little resemblance to a child in our usual sense, but very soon this cluster of constantly dividing cells will begin to have a heartbeat. This will happen in the third week of development, and by the fifth or sixth weeks, the heart rate of the crumbs will be approximately 110 beats per minute ² . As the cardiovascular system develops, these indicators will change even after birth. Amazing, right?

In the third week, he will be just a ball of cells, and it is still completely unclear where he will have a head and where - legs. Only by the 21st day after conception will it be possible to distinguish on his future body the front and back, left and right sides, as well as the top and bottom ³ .

Tiny babies have a tail during their fetal development, which later becomes part of the coccyx. This shows that we are all descended from a common ancestor.

All organs and tissues of the baby are formed from the so-called rudiments. In the third week, not only his heart is formed, but also the future intestines, and the neural tube - the future brain . At day 28 (week 4), the baby is only about 2-3.5 millimeters in size, yet he manages to do an incredible job of growing and developing.

The first trimester is not in vain considered the most important period in the intrauterine development of the baby. At this time, the structures of his body are formed and the so-called critical periods pass:
3rd-16th weeks - the formation of the central nervous system: he has a brain and spinal cord.
3–8 weeks — the formation of the cardiovascular system. Already at the 8th week of pregnancy, the tiny heart looks almost like an adult. It is divided by partitions into four chambers and beats rapidly - the heart rate is up to 177 beats per minute ⁴ .
4-7 weeks - arms and legs appear.
4-8 weeks Ears and eyes appear.
6th-9th weeks - palate and rudiments of teeth appear.

When will the baby's heart beat and what will he look like? The placenta helps with this. And here's what is amazing: that thin thread that will bind you for many months does not appear immediately. It develops, forming from the embryonic membranes.

The placenta is adjacent to the walls of the uterus and, as it were, "grows" into it. Then many new blood vessels form, which will provide blood and nutrients to the baby. This occurs from about the third week and ends by the beginning of the second trimester. Interestingly, the baby's blood and yours do not mix, because the placenta is also a kind of barrier between you.

And quite selective: not every substance is able to overcome it.

Before transferring the necessary compounds to the baby, the placenta can "prepare" them. For example, synthesize proteins or break down fats.

Together with water, electrolytes enter the placenta - potassium, sodium, chlorides, as well as trace elements. The placenta also plays an important role in vitamin metabolism.

Your baby is like a tiny fish swimming in the amniotic fluid. The amnion is involved in their creation - a membrane that forms in the second week after conception, and then rapidly increases, filled with fluid ⁵ . These waters protect the baby from external influences, shocks, and reduce the effect of gravity. The water environment also simplifies the movements of the child - it becomes much easier to make them.

Amniotic fluid also transforms all sounds, as if softening them. So the baby in the next nine months will be in a pleasant twilight, warm and among muffled sounds, such as your heartbeat, the voices of relatives and the noise of the environment. Only your hugs can be more comfortable, but be patient: the pregnancy will fly by quickly.

Stress is not only a change in hormone levels, but also an increased risk of inflammatory reactions in the body. Needless to say, how important it is to avoid it. Whatever happens around you, whatever the storms and thunderstorms, there is nothing more important than a child now in the world. Stop, breathe, think about your small, but already so big happiness and switch to what brings you a good mood.

Is it true that the baby is already moving

Baby's sense organs are formed quite early. For example, in the fourth week, tissues of the ear and lens of the eye are laid, and the outer ear and eyeball - already in the sixth week ⁵ . True, in the first trimester of pregnancy, the baby is not yet ready to experience any sensations - his sensory system is now just a set of receptors, mainly olfactory and visual. But the nervous system can already perform its functions, albeit primitive ones. So, by the eighth or ninth week, the baby's brain is already able to send signals to the limbs - and this is nothing more than the first attempts to move the whole body. Of course, you still don’t feel them because of the small size of the child, but for the development of the nervous system, muscles and bones of the baby, these movements are very important - they train his nervous system, and then the muscles.

Studies show that the nature and intensity of movements are formed even before 10-14 weeks of pregnancy and remain approximately the same throughout the entire period ⁷ . Usually, for a period of about 9 weeks, the movements of the baby are small periods of contractions, and then a long period of rest. By this time, he can roll from side to side, unbend and bend his back and neck, turn his head, swing his arms and legs. In some cases, it was even possible to fix how such a baby bent and then extended his leg ⁷ .

The first movements the child begins to make at the age of seven weeks, and they can even be seen on ultrasound - this is the bending of the body. Still, it will be more clearly noticeable at a slightly later date ⁸ . For example, on the 9th week he already moves his arms and legs, makes swallowing movements, on the 10th week he makes a kind of breathing movements and opens his mouth, and on the 11th he yawns ⁷ .

Drinking and pissing

At the 10th week of pregnancy, the baby begins to swallow amniotic fluid ³ .
Following this, his excretory system almost immediately begins to work: at the 11th week, the kidneys are already filtering urine. After the baby pees, urine enters the amniotic fluid and is excreted through the placenta, which completes its formation by the end of the 1st trimester. This process will be completed completely on the 12th week ⁹ .

At the beginning of pregnancy, amniotic fluid, or amniotic fluid, is formed from maternal plasma, which is part of the blood in the vessels, later - due to the secretions of the baby: through the skin, genitourinary system and lungs. As strange as it may sound, urine is one of the main sources of amniotic fluid. Of course, it differs in composition from the "adult". The intensity of its release also differs. For example, by the end of pregnancy, the baby excretes 400–1200 milliliters of urine per day ¹⁰ .

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