Welcome to the first trimester of your pregnancy! What an exciting journey, especially for those of us who are first-time moms. Undoubtedly, you likely have many questions about what to expect during the entirety of your pregnancy, but let's start with the first trimester. Let us guide you through the development of your baby week-by-week and what you may experience during this time period. The first trimester is full of significant changes—not just for your body but also for your growing baby.
Understanding Gestational Versus Conceptual Age
First, it's important to note that there are two ways of calculating your due date and how far along you are: gestational and conceptual.
Gestational age is the most common method used to measure pregnancy duration. It is calculated from the first day of the woman's last menstrual period (LMP). This method assumes a standard 28-day cycle, with ovulation occurring on the 14th day. Therefore, gestational age includes about two weeks during which a woman is not actually pregnant (before ovulation occurs). In this article, we will be referring to gestational age for the week-by-week development of your baby.
Conceptual age, also known as embryonic or fetal age, measures the age of the embryo or fetus from the time of fertilization. This age is usually about two weeks less than the gestational age since it does not count the initial two weeks before conception.
Not sure which method your doctor is using to determine your due date? Ask!
Weeks 1-4: Early Development
What's happening: As mentioned above, the first two weeks include your last menstrual period before the pregnancy starts, and actual conception typically occurs around the end of the second week.
By the third week of pregnancy, fertilization has likely occurred, marking the beginning of the embryonic stage. The fertilized egg, now called a zygote, will divide several times to form a ball of cells known as a blastocyst. The blastocyst implants itself into the uterine wall. At this point, layers of cells will develop into different organs and parts of the baby's body start to form, though the brain's development has not yet begun.
In the fourth week, a structure called the "primitive streak" forms in the embryo. This structure eventually develops into the neural tube, from which the brain and spinal cord (central nervous system) will develop. Cells begin to specialize and organize into areas that will become various tissues and organs. By the end of week four, the neural tube begins to form, laying the foundational cells that will grow into the brain and spinal cord, including the brain stem.
Common concerns: You might not know you're pregnant yet, but hormonal changes can already start affecting your body. Early signs can include mild cramping and spotting, known as implantation bleeding.
A common concern we often hear is, "Oh, no, I had a couple of glasses of wine before I knew I was pregnant! Did it affect my growing baby?" The good news is that your growing baby is likely okay; it's important to quit drinking alcohol, smoking, or the use of illegal drugs as soon as you know you are pregnant. Consult with your doctor as soon as possible on any other medications, prescribed or over the counter, to ensure they are safe to take while pregnant.
Weeks 5-8: Major Growth Spurts
What's happening: This period is crucial for the development of your baby's major organs. In week 5, The heart begins to form and will start beating by the end of this week. Initially, it might be just a simple, single chamber that starts pulsating. As you head into week 6, basic facial features begin to form, and areas that will become the eyes and ears start as small depressions on the sides of the head. Additionally, small buds appear that will eventually grow into arms and legs. As you progress through week 7, the baby's brain continues to grow, facial features further define, the limbs extend, and paddle-like hands and feet begin to emerge. By the end of week 8, the embryo starts to look more like a human, with arms, legs, fingers, and toes starting to form. The heart, which has already started beating, will become more defined and audible via ultrasound.
Common concerns: Nausea often starts around the sixth week. Despite being commonly called "morning sickness," it can occur at any time of the day. Learn more about morning sickness and treatment options here. Fatigue is also common due to increased levels of progesterone and your body's intensified effort to nurture the embryo. Your body is going through a lot of changes, and there are a number of symptoms that you may experience during this time, such as cravings and increased urination. Always consult your doctor to ensure that your symptoms are "normal" or seek additional treatment if symptoms are disrupting daily life.
Weeks 9-12: Systems Start Functioning
What's happening: Your baby is now officially called a fetus. Vital organs, such as the heart, kidneys, liver, and brain, continue to develop and begin functioning. The heart has now been divided into four chambers. The eyes are fully formed but remain closed. The earlobes, upper lip, and nose tip are becoming more distinct. The external genitals are also developing, though it's usually too early to determine the sex via ultrasound. The placenta is now fully formed and takes over hormone production, which can mean some of your early symptoms might start to decrease. During this time, your baby will even start to move. You likely won’t be able to feel it yet but know your baby is beginning to bend and twist.
Typically sometime between weeks 8-12, an ultrasound may be scheduled so you can officially see your new baby - how exciting!
Common concerns: Emotional fluctuations are common; thank your surging hormones. Some women continue to experience morning sickness, while others may start to feel more energetic.
What Can You Do to Stay Healthy in the First Trimester?
- Regular Prenatal Visits: Keep up with your scheduled appointments. These visits are crucial for monitoring your health and the baby's development. You may be surprised, but with most low-risk pregnancies, your first trimester will not include many doctor visits.
- Balanced Diet: Eat a variety of healthy foods. Include vegetables, fruits, whole grains, calcium-rich foods, and proteins.
- Supplements or Prenatal Vitamin: It is recommended to take a daily prenatal vitamin with DHA, which helps with baby’s brain development. Consult your doctor if you have questions about supplements or recommendations on taking a prenatal vitamin.
- Stay Hydrated: Drink plenty of fluids. Water is best, but small amounts of milk, coffee, tea, and juice are okay.
- Exercise: Generally, most women are advised to continue their normal exercise regimen throughout pregnancy. However, you should consult your doctor first. You can find more information on exercising while pregnant here.
- Rest: Listen to your body. If you're tired, rest. Getting enough sleep helps.
Common Questions Answered
- Is spotting normal? Light spotting can be normal, but always report any bleeding to your doctor.
- What about medications? Discuss any medications you are taking with your doctor to ensure they are safe during pregnancy.
- Can I still have caffeine? Limit caffeine intake to less than 200 mg per day, the equivalent of about one 12-oz cup of coffee.
- Is it normal to feel this tired? Fatigue is very common in the first trimester due to hormonal changes, especially an increase in progesterone, and your body working harder to support the developing fetus.
The First Trimester and Two More To Go!
The first trimester is both thrilling and challenging. Remember, every pregnancy is unique, so it's important to focus on your own journey and needs. If you have any concerns or need advice, don't hesitate to ask your doctor. We are here to support you every step of the way on your path to motherhood! Looking for more information past the first trimester? Check out our article on the full week-by-week development of your baby.
Learn what to expect in your second trimester of pregnancy.