First Trimester of Pregnancy: 1-12 Weeks

Welcome to the first trimester of pregnancy. You’re embarking on a remarkable journey, filled with new experiences and questions about what to expect in the first trimester weeks ahead. So, what exactly is the first trimester of pregnancy? This guide covers essential aspects of the first trimester, including its timeline, foetal development, early pregnancy symptoms and tips to navigate these initial weeks.

Highlights From the First Trimester of Pregnancy

1st Trimester Highlights
Foetal Development
  • From a cluster of cells to an embryo and then a foetus, your baby undergoes significant changes.
  • The formation of the brain, spinal cord, heart and small limbs marks this period.
Potential Symptoms
  • Notable changes in breasts and skin
  • Increased fatigue
  • Nausea or morning sickness
  • Mood fluctuations
1st Trimester TipEmbrace healthy lifestyle choices – balanced nutrition, regular physical activity, and avoiding smoking and alcohol are crucial.



First Trimester Weeks: How Long Is the First Trimester and When Does It End?

Common questions include, ‘How long is the first trimester?’, ‘How many weeks is the first trimester?’ and ‘When does the first trimester end?’ The first trimester of pregnancy spans until the end of week 12 and begins even before conception. It’s calculated from the first day of your last menstrual period (LMP). With a standard pregnancy lasting about 40 weeks, your GP estimates your due date 40 weeks from your LMP. Haven't seen your GP yet? Try our Due Date Calculator for an early estimate. It’s important to remember that most babies don’t arrive exactly on their due date, which is just an estimate. They’re typically born within two weeks before or after the estimated date. A pregnancy is considered full-term from 37 weeks. Each of the three trimesters in pregnancy contributes to the 40-week duration. Explore each week in detail, including those in the first trimester, in our comprehensive pregnancy calendar article. Though it might be early for your baby’s gender reveal, have fun guessing with our non-scientific Chinese Gender Prediction tool:

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Your Baby’s Development in the First Trimester

The first trimester sees your baby evolve from a tiny cell bundle to an embryo, and eventually a foetus the size of a large plum.

In these initial months, the brain, spinal cord, heart, and tiny limbs, including fingers and toes, begin to take shape.Your uterus prepares a nurturing environment, supported by the evolving placenta and umbilical cord.

By the end of your third month, all vital organs and body parts will be formed, albeit very small.

The first trimester is a period of rapid and fascinating foetal development. Continue reading to discover key milestones during these early weeks.

4 Weeks: Implantation

At this stage, the rapidly dividing cell cluster, known as a blastocyst, implants itself in the uterus. It’s during this time in the first trimester that some pregnant people might notice implantation bleeding, typically characterized by light spotting.

When implantation happens, the outer cells start to form connections with your blood supply, while the inner bundle of cells splits into two then three layers of cells that will form the various parts of your little one’s body:

  • The innermost layer of cells will become the lungs, stomach and other parts of the digestive and breathing system

  • The middle layer will eventually form the muscles, bones, heart and circulatory system

  • The outer layer becomes the brain and nervous system, skin, nails, tooth enamel and eye lenses.

6 Weeks: Taking Shape

By 6 and 7 weeks, significant developments occur. The heart, lungs and other vital organs start their formation. Alongside this, the embryo’s head and limbs begin to emerge. The transformation from a cell cluster to a more defined C-shape is remarkable during these first trimester weeks.

9 Weeks: In Motion

At nine weeks your little one is now known as a foetus, which just means ‘offspring’ in Latin. Your little one’s face is forming. The eyes are bigger and starting to gain some colour and your foetus now has a mouth and tongue, complete with taste buds. The hands and feet are taking shape, too, although at this time there are only grooves where the fingers and toes will be.

10 Weeks: Shaking and Moving

At this stage of the first trimester, your little one is moving around. These jerky movements are visible on an ultrasound scan at about 10 weeks pregnant, but you probably won’t feel any movement yourself until sometime around or after 16 weeks. Keep in mind that nobody can tell just when those first flutters will come, and it may be a little later for you – especially if you’re a first-time parent-to-be. Experts say feeling foetal movement for the first time can happen anywhere from 16 to 24 weeks. Around this week, the facial features are also becoming more prominent. There are two tiny slits which are the nostrils and the upper lip is in place. Amazingly, your little one’s growing jawbones are already home to what will eventually become baby teeth.

10 to 12 Weeks: Heart to Heart

Your little one’s heart is fully developed once you’re 11 weeks pregnant, and it’s pumping quickly – at a rate of around 180 beats per minute. This is probably two or three times as fast as yours, although your own pulse is sure to quicken if you glimpse your little one at your next ultrasound scan.

Illustration of Foetal Development Week by Week

To visualise the astonishing changes, take a look at the illustration below. It provides a week-by-week overview of your baby’s growth during the first trimester.

What’s in Store for You in the First Trimester?

During the first trimester, you’re likely to encounter various changes, make some decisions, and experience new sensations. Here’s what you might expect:

  • Confirming your pregnancy. Experiencing early pregnancy symptoms but unsure if you’re pregnant? A home pregnancy test can offer confirmation, but it’s wise to visit your GP for certainty.

  • Determining your due date. Once pregnancy is confirmed, naturally, you'll be keen to know the arrival date of your baby. Our Due Date Calculator can estimate your due date based on your last menstrual period or conception date. Your GP or midwife will also provide an estimated due date.

  • Announcing your pregnancy to your partner. If your partner is not yet in the know, finding creative ways to break the news can be exciting. Here are some pregnancy announcement ideas for telling your partner, whether humorous, romantic or inventive.

  • Deciding when to announce to others. Choosing when to share your pregnancy news with family and friends is personal. Many prefer waiting until the end of the first trimester or the beginning of the second. For workplace announcements, your GP can offer tailored advice.

  • Navigating emotional changes. The first trimester of pregnancy can be emotionally challenging due to fluctuating pregnancy hormones. Feeling differently than expected about your pregnancy is completely normal. We delve into these emotional shifts and offer coping strategies in the following section.

  • Managing pregnancy symptoms. The first trimester may bring some uncomfortable symptoms. Remember, these common first trimester signs and symptoms signify the development of new life and that your pregnancy is going well. These early symptoms often subside in the second trimester. If you have any concerns about what you’re feeling, contact your doctor or midwife.

  • Your first antenatal checkup. Schedule your initial visit with your doctor or midwife as soon as you discover you're pregnant. This visit is vital for monitoring both your health and your baby's progress, and to ensure your baby's development is on track.

  • Undergoing tests and ultrasounds. The first trimester may involve at least one ultrasound scan, offering a more precise due date and pregnancy week estimation. Screening tests for genetic conditions, including blood tests and ultrasound scans, are also available, though optional. Your doctor or midwife will discuss the options, risks and benefits to help you make an informed decision.

  • Discovering a multiple pregnancy. At your first first trimester ultrasound, you might learn you’re expecting twins or more! If so, check out our FAQ on multiple pregnancies.

  • Becoming informed. Especially for first-time parents, researching pregnancy, childbirth and child development can be reassuring. The Pampers website is a treasure trove of information and tips covering the first three years of your child's life.

  • Understanding second pregnancy differences. If this isn't your first pregnancy, you might notice differences. Our article on second pregnancies provides insights into what might change this time around.

  • Adopting healthier habits. Making healthy lifestyle adjustments during the first trimester benefits both you and your baby. Discuss nutritional needs and prenatal vitamins with your doctor or midwife. Limit your caffeine intake and avoid alcohol, smoking and drugs. Get moving after checking with your doctor what exercise is safe for you to do during pregnancy.

  • Prioritising rest and relaxation. Creating new life is exhausting work. With two more trimesters ahead, it’s crucial to conserve energy and rest when you can.

What Weeks and Months Are in the First Trimester?

Wondering about the first trimester weeks and months? Although 40 weeks don't neatly divide into nine months, the first trimester is typically about three months or 12 weeks long, as mentioned earlier. Explore the visual below to see the progression and get an idea of your growing belly:

First Trimester Symptoms

Every pregnancy is distinct, yet there are common first trimester symptoms experienced by many during this period. Here’s what you might encounter:

  • Emotional ups and downs. This journey’s initial stage is a whirlwind of emotions, intensified by the surge of hormones during early pregnancy. It’s normal to have mixed feelings about the pregnancy, from joy to worry. Sharing these emotions with others or seeking professional guidance can be beneficial.

  • Changes in breasts and skin. Your body undergoes numerous changes due to pregnancy hormones. Expect heavier and tender breasts, more visible veins and potentially darker skin, moles or nipples. These changes typically fade post-delivery.

  • Fatigue. Tiredness is a hallmark of the first trimester, as your body adjusts hormonally. Rest, a balanced diet, and gentle exercise can alleviate fatigue.

  • Nausea. Often referred to as morning sickness, nausea (and sometimes vomiting) can occur any time of day. Lifestyle adjustments can help manage morning sickness, such as avoiding triggers and opting for bland, small meals.

  • Cravings. Unusual food cravings are generally harmless but consult your doctor or midwife if you crave non-food items.

  • Frequent urination. Hormonal changes and increased organ activity can lead to more frequent bathroom visits.

  • Acne. Hormone-induced oil production can cause breakouts. Maintain a skincare routine with mild cleansers and oil-free products.

  • Bloating. Hormones can also cause bloating, often mistaken for an early pregnancy belly.

  • Thicker, shinier hair. Some pregnant people find that the extra oestrogen coursing around their bodies makes their hair more luxuriant in the first trimester. This could be one of the more welcome symptoms of pregnancy!

If you’re concerned about your weight during pregnancy, your doctor and midwife can help you stay on track and let your know what a healthy weight is for you. You can also try our Pregnancy Weight Gain Calculator for a little extra guidance:

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Precautions to Take During the First Trimester of Pregnancy

It’s crucial to prioritise your health and your baby’s during the first trimester. Consider the following precautions:

  • Avoid alcohol, tobacco, secondhand smoke and illicit substances.

  • Limit caffeine intake to less than 200 mg per day.

  • Steer clear of certain foods like raw seafood, raw eggs, unpasteurised milk and soft cheeses.

  • Consult your doctor or midwife before taking any medication.

  • Be aware of environmental hazards, such as certain cleaning products or workplace materials.

  • Minimize exposure to radiation, including medical X-rays.

  • Engage in safe, gentle exercise, after consulting with your doctor or midwife.

  • Avoid saunas and hot tubs due to high temperatures.

  • Reach out to your doctor or midwife with any concerns or questions during this crucial period.

When to Contact Your Doctor

Stay in regular contact with your doctor or midwife, especially during the first trimester. Attend all antenatal checkups, and be vigilant about certain symptoms that may require immediate attention:

  • Severe abdominal pain

  • Heavy bleeding or spotting

  • Severe dizziness or fainting

  • Sudden severe swelling

  • Persistent headaches

  • High fever or chills

  • Breathing difficulties

  • Continuous vomiting

  • Signs of dehydration

  • Blurred vision or spots before your eyes

  • Any unusual or concerning symptoms.

Your doctor and midwife are valuable resources for any concerns during the first trimester.


The first trimester often feels like a mix of fatigue, emotional fluctuations and physical changes like nausea and breast tenderness.

Checklist for the First Trimester

Here’s a checklist for your first trimester:

  • Ask your doctor what tests or scans are recommended for you based on your medical history, and mark these in your calendar. Think about whether you’d like any additional optional genetic screening or diagnostic tests.

  • Ask your GP when you can get the flu shot.

  • Consult your doctor about taking folic acid and other prenatal vitamins.

  • Ask your doctor if the medications you currently take are safe during pregnancy.

  • Though this is a rare condition, you may want to read up on the signs of an ectopic pregnancy just in case.

  • Read up on maternity leave and paternity leave.

  • Find out what free medical care you are eligible for under the NHS. For example, you may be able to get free prescriptions and dental care among other benefits.

  • Make an appointment with your dentist to ensure you get good dental care during your pregnancy.

  • As your breasts grow, go for a bra fitting at your local retailer to ensure you’re in the right size.

The Bottom Line

The first trimester of pregnancy brims with excitement and anticipation. By adhering to our advice and your doctor and midwife’s recommendations, you can navigate this stage with greater ease. It’s essential to listen to your body, engage in self-care, keep a transparent dialogue with your doctor or midwife and promptly seek guidance for any concerns or questions. You’re embarking on a wondrous journey. Now is the perfect time to cultivate a supportive and nurturing environment, fostering the growth and development of your baby.

How We Wrote This Article The information in this article is based on the expert advice found in trusted medical and government sources, such as the National Health Service (NHS). You can find a full list of sources used for this article below. The content on this page should not replace professional medical advice. Always consult medical professionals for full diagnosis and treatment.

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