First Trimester of Pregnancy: 0-13 Weeks

Welcome to the first trimester of pregnancy. This is the start of a wonderful journey, and it’s natural to have lots of questions about what’s in store for you in the weeks and months ahead. We’ve compiled some essential information on fetal development during the first trimester, common early pregnancy symptoms, and what’s coming your way this trimester.

How Long Is the First Trimester?

The first trimester is about 13 weeks long, and it actually starts before you become pregnant. This is because your estimated due date is usually calculated from the first day of your last menstrual period (LMP).

A full-term pregnancy is roughly 40 weeks long, so your healthcare provider will calculate 40 weeks from the start of your last period to estimate when you’re due.

Keep in mind that most babies aren’t born exactly on their due date, which are only estimates. Rather, they are typically born in the two week period before or after the date. Your pregnancy is considered full term pregnancy at the start of 39 weeks.

The trimesters of pregnancy are composed of 40 weeks. 

The weeks of pregnancy are sometimes broken down into months, but there’s some variance in how the weeks are grouped into months. This is because 40 weeks don’t divide neatly into 9 months, and because months typically are a little longer than 4 weeks.

Your Baby’s Development in the First Trimester

During the first trimester of pregnancy, what starts as a tiny bundle of cells quickly turns into an embryo and then a fetus that's about the size of a large plum.

In these first few weeks of pregnancy, your little one’s brain, spinal cord, heart, and tiny limbs — complete with fingers and toes — will form.

Meanwhile, your uterus is becoming a comfortable home for your little one, who will be nourished by the developing placenta and umbilical cord.

By the end of third month of pregnancy, all essential organs and body parts will be in place — albeit in teeny-tiny size.

There’s a lot happening in the first trimester. Here are a few of the most exciting fetal development milestones:

4 Weeks: Implantation

The ball of rapidly dividing cells, called a blastocyst, implants in the uterus. This is when some moms-to-be experience implantation bleeding.

The inner cells will become the embryo and the outer cells will become the placenta. The placenta will provide nourishment to your baby from now until delivery.

6 Weeks: Taking Shape

Around six and seven weeks, the heart, lungs, and other key organs start to develop, and the head and limbs also take shape.

What was a ball of cells just weeks ago is forming a more recognizable c-shape.

9 Weeks: In Motion

From around 9 weeks, your little one, now known as a fetus, may start moving around.

However, you probably won't be able to feel any movement until the second trimester. For more on this topic, read up on quickening and fetal movement.

10 Weeks: Fingers and Toes

This week your baby's fingers and toes lose their webbing and continue to grow longer. It won't be long until you're able to count them!

10 or 11 Weeks: The Sound of Your Baby’s Heart

From around 10 or 11 weeks, your little one’s heart may be beating loudly enough to be heard with a Doppler ultrasound, which your healthcare provider may perform at your next appointment.

Illustration of Fetal Development Week by Week

Click through the illustrations below for a visual representation of how your little one develops week to week during the first trimester:

fetal development visuals carousel

What’s in Store for You This Trimester

Here are just some of the things you may be doing or that may be coming your way in the first trimester:

  • Confirming your pregnancy. If you’re experiencing early pregnancy symptoms, but are still not sure if you're actually pregnant, a home pregnancy test can confirm your hunch, but head to your healthcare provider if you’re in any doubt. If you haven't found a prenatal healthcare provider yet, here are some tips to help you find someone.

  • Announcing your pregnancy to your partner. If your partner doesn’t already know you’re pregnant, you may be looking for fun ways to reveal the special news. Here are nine ways to tell your partner you’re pregnant. Whether you go with something funny, romantic, or downright creative, your partner will love your surprise!

  • Considering when to tell others. When to announce your pregnancy to loved ones is a personal choice, but some moms-to-be prefer to wait until the end of the first trimester or the start of the second to share the news. If you’re wondering when to tell your colleagues or employer, your healthcare provider can give you personalized advice.

  • Experiencing shifting emotions. Some moms-to-be find the first trimester of pregnancy quite challenging. Pregnancy hormones can trigger more intense mood swings than you may be used to. Feeling differently about your pregnancy than what you thought you would is also perfectly natural. We cover changing emotions and how to cope with mood swings in the next section.

  • Working through pregnancy symptoms. You may have to deal with some annoying first trimester symptoms — more on these below. The good news? Each symptom is a reminder that you’re bringing a new life into the world, and each week of pregnancy brings something interesting and new. It may also help to remind yourself that these symptoms won’t last forever. In fact, it’s common for the early symptoms of pregnancy to subside in the second trimester, and this may even be accompanied by a burst of extra energy.

  • Going to your first prenatal care checkup. As soon as you know you are pregnant, schedule your first visit with your healthcare provider. You’ll have many checkups during your pregnancy so your provider can monitor your and your baby’s health. At this first visit, your provider will ask you lots of questions about your medical history, run tests and do a physical exam, and give you an estimate of your due date. Your provider will also be able to give you an idea of when future checkups and exams will take place. Prepare a list of questions beforehand so you come away from this first checkup with all your questions answered and who to call outside of hours in case you have an urgent question or concern.

  • Having tests and ultrasounds. In the first trimester you may be offered at least one ultrasound scan. Using ultrasound, your healthcare provider can give you a more accurate estimate of your due date, and how many weeks pregnant you are. You may also be offered screening tests for certain genetic conditions, and these tests typically include a blood test and an ultrasound scan. These screening tests are optional, and it's important to note that they don't diagnose any particular conditions; rather, they evaluate the possible risk of various conditions. After a screening test, you may choose to have more thorough diagnostic tests done. Which if any screening and diagnostic tests to do is a personal choice. Your provider can explain your options to you, including any risks and benefits, so you can make an informed choice.

  • Learning you’re having twins. There’s a small chance that you’ll come away from a your first trimester ultrasound with a little more than you bargained for. Typically, an ultrasound can show if you’re pregnant with twins, triplets, or more by the time you’re 12 weeks pregnant. If you do happen to be pregnant with more than one, read our FAQ on twin pregnancy for the lowdown on twin pregnancy.

  • Becoming informed. If this is your first baby, doing some research on pregnancy, childbirth, and child development might help you feel calmer and in control of what’s to come. In addition to reading up on these topics, you might also like to speak with other moms in your area, who will have lots of personal insights and valuable experience to share. The Pampers website is packed with information and tips about pregnancy and the first three years of your little one’s life. To get started now, check out warning signs not to ignore during pregnancy and take a deep dive into your pregnancy with our week-by-week pregnancy guide.

  • Discovering how a second pregnancy may be different. If this isn’t your first pregnancy, you may be wondering how things could be different this time round. Read our article on differences you may experience during a second pregnancy.

  • Making some healthy lifestyle changes. Adopting healthier habits now is important not only for you but your baby as well. Eat a healthy, balanced diet to help with morning sickness and cravings, and to keep your pregnancy weight gain on track. Speak to your healthcare provider about your calorie and nutritional needs during pregnancy and ask whether you need to take prenatal vitamins. Alcohol, smoking, and drugs can be harmful, so it’s best to steer clear of these. As for those life-saving coffees (and chocolate bars), the recommendation is to limit your caffeine intake to 200 milligrams a day, the equivalent of a 12-ounce cup of joe. Start exercising or keep up with your exercise routine after checking with your provider to learn what exercise is safe for you to do during pregnancy.

  • Resting and relaxing. Building a new life takes a lot of energy. And, you've got two more trimesters to go before you'll meet your baby, so conserve your energy.

First Trimester Symptoms

Although every pregnancy is unique, here are some of the most common symptoms of the first trimester:

  • Emotional ups and downs. The first stage of this journey may be a bit of a rollercoaster ride, but that’s hardly surprising as something amazing is happening in your belly and you’re at the start of a new journey. The rush of hormones in early pregnancy can also trigger more intense mood swings than you may be used to. Don’t be surprised if you feel a little differently about being pregnant than you had expected. Aside from feelings of joy and excitement, you might also have worries or concerns about how pregnancy and having a baby will change your life. It’s helpful to talk about these feelings, either with a professional or with your support network of friends and family.

  • Changes in breasts and skin. Pregnancy hormones could be making your breasts heavier, and a little sore or tender. With all that extra blood to carry around your body, your veins may be more visible through your skin. Meanwhile, those hormones may also make your skin, moles, and birthmarks or your nipples a little darker. Most of these changes gradually fade away after you give birth.

  • Fatigue. Feeling tired or exhausted is particularly common in the first trimester, as your hormones go into overdrive. The best thing you can do if you’re experiencing pregnancy fatigue is to get plenty of rest. Maintaining a healthy diet and doing gentle exercise might also help you feel better.

  • Nausea. The queasiness (and sometimes vomiting) known as morning sickness may appear in the first trimester. Contrary to its name, though, it doesn’t strike only in the mornings! Try to think of morning sickness as a reassuring reminder that you are pregnant. You might be able to ease some of the symptoms with a few lifestyle changes, like avoiding food or smells that trigger your nausea, and eating smaller, more frequent meals of plain, low fat foods. You may find cold foods easier to stomach than hot meals. Food or drink that contains ginger may also help take the edge off your queasiness.

  • Cravings. Weird as they are, cravings for unusual foods are usually nothing to worry about. However, talk to your healthcare provider right away if you start to crave non-food items like soil or paper.

  • Frequent urination. With rapid hormonal changes and your body’s organs working harder than usual, you may find yourself needing to pee more often than usual.

  • Acne. An increase in oil production triggered by hormones can clog pores and lead to acne in some moms-to-be. Wash your face twice a day, go for oil-free cosmetics, and ask your healthcare provider about any medications that can help reduce breakouts.

Checklist for the First Trimester

  • Find a good prenatal healthcare provider who can help you throughout your pregnancy and during childbirth. Make your first appointment as soon as you know you're pregnant.

  • Ask your provider what tests or scans are recommended for you based on your medical history, and mark these in your calendar.

  • Find out how you can get the flu shot.

  • Consider whether genetic tests like nuchal translucency screening and chorionic villus sampling (CVS) are tests you may like to have.

  • Consult your healthcare provider about taking folic acid and prenatal vitamins.

  • Ask your healthcare provider 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 and symptoms of an ectopic pregnancy just in case.

  • Find out what maternity or paternity leave, you and your partner may be eligible for.

  • Review your health insurance policies to check you have the coverage you want and need. 

  • 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.

  • Start a pregnancy journal or memory book if this is something you think you would like to look back on in the years to come.

  • If you would like to, take your first belly picture and decide on which day of each month you would take your progress photos.

  • Start brainstorming name ideas and start putting a short list of baby names together.

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