Your Pregnancy Week-by-Week: What’s Happening Inside Your Body

As you embark on this incredible journey, you’re probably eager to understand the remarkable changes taking place inside you. Every week brings new developments in your baby’s growth, and being informed can make a significant difference in your experience.

Over the next 40 weeks, your baby will transform from a single cell to a fully-formed infant. Understanding these changes can help you connect with your developing baby and make sense of the physical symptoms you’re experiencing.

Key Takeaways

• You’ll gain a deeper understanding of your baby’s development week by week.
• This guide breaks down each trimester into monthly and weekly milestones.
• You’ll learn how to connect with your developing baby and understand your body’s changes.
• The guide provides valuable insights into your baby’s growth and your body’s adaptation.
• You’ll be better prepared for the physical symptoms and changes you experience during pregnancy.

Understanding Pregnancy Timeline

The journey of pregnancy is typically measured in weeks, not months, and spans approximately 40 weeks. This duration is calculated from the first day of your last menstrual period (LMP), which might seem unusual since you’re not actually pregnant during the first two weeks. However, this method is used because it’s challenging to pinpoint the exact date of conception.

How Pregnancy Is Calculated

Pregnancy duration is calculated as 40 weeks or 280 days from the first day of your last menstrual period. Your healthcare provider will use this date to determine your due date. Although conception typically occurs about two weeks after your last period, this calculation method helps in estimating the gestational age of the fetus.

The Three Stages of Fetal Development

Fetal development progresses through three stages: germinal (conception to week 2), embryonic (weeks 3-8), and fetal (week 9 until birth). Understanding these stages and how pregnancy is calculated can help you make sense of your prenatal appointments and the developmental milestones discussed by your healthcare provider.

Before You Begin: Conception and Fertilization

Understanding how conception works is essential for grasping the early stages of pregnancy. Conception is a complex process involving the union of a sperm and an egg, leading to the formation of a zygote that eventually develops into an embryo.

How Conception Works

Conception occurs when a sperm successfully fertilizes an egg during the fertile window of your menstrual cycle, typically within 24 hours after ovulation. This process begins with ovulation, where a mature egg is released from the ovary into the fallopian tube.

If sperm is present in the fallopian tube, it can fertilize the egg, resulting in conception. The fertilized egg, now called a zygote, starts dividing and travels toward the uterus.

The Journey from Egg to Embryo

After fertilization, the single-celled zygote undergoes multiple cell divisions as it travels through the fallopian tube toward the uterus. By the time it reaches the uterus, it has developed into a blastocyst, a hollow ball of cells with an inner cell mass that will become the embryo and an outer layer that will form the placenta.

The blastocyst then implants into the uterine lining, where it continues to grow and develop throughout your pregnancy. Successful implantation triggers the production of human chorionic gonadotropin (hCG), the hormone detected by pregnancy tests.

By understanding the conception process, you can appreciate why timing is crucial when trying to conceive and why implantation is a critical milestone in early weeks of pregnancy, particularly during the first week after conception.

First Trimester: Weeks 1-12

The initial 12 weeks of pregnancy, known as the first trimester, are crucial for your baby’s development. During this period, your baby grows from a microscopic cluster of cells to a recognizable human form about 2-3 inches long by the end of week 12.

Major Developmental Milestones

Your baby’s major organs and body systems begin to form during the first trimester. As stated by medical professionals, “The formation of the neural tube, heart, limbs, and facial features are significant milestones during this period.“

The first trimester is a critical period of rapid growth and development for the fetus, with most major organs and body systems beginning to form.

Some key developments include the formation of the neural tube, which eventually becomes the brain and spinal cord, and the development of the heart and the beginning of its function.

Common Body Changes You May Experience

You may experience significant hormonal changes that can cause symptoms like morning sickness, fatigue, and breast tenderness. As your body adjusts to the pregnancy, you might also notice frequent urination.

As your pregnancy progresses through the first trimester, it’s essential to be aware of these changes and discuss any concerns with your healthcare provider.

Month 1 (Weeks 1-4): The Beginning

Month 1 of pregnancy, spanning weeks 1 through 4, is a critical period where the foundation for your baby’s development is laid. Although it might seem counterintuitive, the first two weeks of this period are more about preparation than actual pregnancy.

Weeks 1-2: Preparing for Pregnancy

During weeks 1 and 2, your body starts to release more hormones, and your uterus prepares for a potential pregnancy. At the end of week 2, your ovary releases an egg through a process known as ovulation. If sperm meets the egg shortly after ovulation, the process towards pregnancy continues.

Week 3: Fertilization

In week 3, fertilization occurs. A sperm and egg join to create a zygote. This tiny cell begins to divide rapidly as it travels through the fallopian tube towards the uterus.

Week 4: Implantation

By week 4, the developing embryo, now called a blastocyst, implants into your uterine lining. This triggers the production of pregnancy hormones that prevent your next period. The placenta begins to form, creating a vital connection that will provide oxygen and nutrients to your baby throughout the pregnancy. An amniotic sac also forms around the blastocyst, providing a protective environment.

Some key developments during these weeks include:

• The fertilized egg (zygote) begins dividing rapidly as it travels through the fallopian tube toward the uterus during week 3.
• By week 4, the blastocyst implants into your uterine lining, triggering the production of pregnancy hormones.
• The placenta and amniotic sac begin to form, laying critical foundations for your baby’s development.
• Though your baby is only about 2 millimeters long by the end of week 4, significant developmental milestones are achieved.

You might experience early pregnancy symptoms during week 4, such as light spotting from implantation, fatigue, or breast tenderness. However, many women do not notice any symptoms yet.

Month 2 (Weeks 5-8): Rapid Development

The second month of pregnancy is a period of rapid development for your baby. During this time, many people realize they’re pregnant due to the surge in pregnancy hormones, and by around the fifth week, an at-home pregnancy test will likely show a positive result.

Week 5: Neural Tube Formation

In week 5, the neural tube forms, which is the foundation of your baby’s brain, spinal cord, and nervous system. The tiny “heart” tube starts to pulse around 110 times a minute by the end of this week. Folic acid supplementation is crucial during this period to support the development of the neural tube.

Week 6: Heart Development

By week 6, your baby’s heart begins beating and can often be detected on a vaginal ultrasound. Tiny buds that will become arms and legs also develop, and structures that will form the ears, eyes, and mouth start to take shape.

Weeks 7-8: From Embryo to Fetus

During weeks 7-8, your baby undergoes significant development. Bones begin replacing soft cartilage, and genitals start to form. By the end of week 8, your baby is about 1 inch long and is now referred to as a fetus. The umbilical cord is fully developed, efficiently delivering nutrients and oxygen while removing waste products.

As your baby grows, you may start experiencing pregnancy symptoms like morning sickness, fatigue, and breast tenderness due to rising hormone levels. It’s an exciting time, and understanding these developments can help you better appreciate the changes happening within your body.

Month 3 (Weeks 9-12): Completing the First Trimester

You’re now in the final stretch of your first trimester, and your baby is growing rapidly. This period is crucial as your embryo officially becomes a fetus, marking significant developmental milestones.

Week 9: Facial Features Form

During week 9, your baby’s facial features become more defined. The beginnings of teeth and taste buds are forming, and its muscles are developing, giving its body a more human appearance. Although its head is still relatively large, comprising about 50% of its length, your healthcare provider may be able to detect its heartbeat using a Doppler ultrasound.

Week 10: Limb Development

By week 10, your baby’s arms, hands, fingers, feet, and toes are fully formed, with no webbing between the fingers and toes. Fingernails and toenails start to develop, and the external ears form. The external genitals also begin to form, although they are not yet visible on an ultrasound.

Weeks 11-12: Organ Systems Begin Functioning

In weeks 11-12, your baby starts to make spontaneous movements, such as opening and closing its mouth and fists. Although it’s too early to feel these movements, its knees, elbows, and ankles are working. The bones are hardening, but the skin is still transparent. By the end of week 12, all major organ systems are functioning, and the baby measures about 2.5-3 inches, weighing approximately 1 ounce.

Month 4 (Weeks 13-16): Visible Growth

During weeks 13-16, your baby undergoes substantial changes, both in form and function. This period is crucial as your baby’s features become more defined, and significant developmental milestones are achieved.

Vocal Cords and Skin Development

In weeks 13-14, your baby’s vocal cords form, and their large head begins to grow proportionate to their body. By week 14, the skin starts to thicken, and fine hair called lanugo begins to grow, helping to regulate body temperature. The external genitals are fully developed, and fingerprints start to form.

Hearing and Movement

By weeks 15-16, your baby’s ears have moved to their final position, enabling them to hear sounds from inside your body, such as your heartbeat. Your baby now makes more coordinated movements, including sucking their thumb and practicing breathing movements. By week 16, they can even react to light by turning away from it.

Your baby is now about 4-5 inches long and weighs approximately 3-4 ounces, roughly the size of an avocado. Many women start showing a noticeable pregnancy bump during this month, especially in subsequent pregnancies.

Month 5 (Weeks 17-20): Feeling Movement

You’re halfway through your second trimester, and it’s likely you’ll start feeling your baby’s kicks and jabs. During this period, significant developments occur, and your baby becomes more active.

Weeks 17-18: Protective Coating and Sleep Cycles

At week 17, your baby’s skin is still thin but starts to put on fat, and it’s covered with a protective whitish coating called vernix caseosa. By week 18, your baby is covered in lanugo, a fine hair that helps keep them warm. Your baby may also develop sleep-wake cycles and can be woken up by loud noises.

Weeks 19-20: Kicks and Sensory Development

By week 19, your baby is getting stronger, and you may feel kicks and punches. Your baby now has unique fingerprints and can hiccup. At week 20, your baby’s nails grow towards the end of their fingers, and the area of the brain responsible for their five senses begins to develop. You might also undergo an anatomy scan ultrasound around this time, providing detailed images of your baby’s development.

Your baby is now about 6-7 inches long and weighs approximately 9-11 ounces. As your baby grows, their movements become more pronounced, and you may start to feel more distinct kicks and jabs.

Month 6 (Weeks 21-24): Increasing Viability

The sixth month of pregnancy is crucial for your baby’s development, with major advancements in physical and sensory capabilities. During this period, your baby’s movements become more pronounced and coordinated.

Coordinated Movements

Between weeks 21 and 22, your baby’s limb movements become more frequent and coordinated. The fetus can now touch its face, grasp the umbilical cord, and respond to external sounds. Your baby’s hearing also becomes more sophisticated, detecting your heartbeat, digestive sounds, and even external noises like music or your voice.

Lung Development

Weeks 23 and 24 mark a significant milestone in lung development as surfactant production begins. This critical development helps the air sacs in the lungs remain open after birth, enhancing the baby’s chances of survival if born prematurely. By week 23, with intensive medical care, babies may survive outside the womb, though they face considerable challenges.

During this month, your baby starts accumulating fat under the skin, which aids in temperature regulation after birth and reduces the wrinkled appearance. The brain continues its rapid development, with billions of neurons forming and creating connections that support learning and development.

By the end of month 6, your baby measures about 11-14 inches long and weighs approximately 1-1.5 pounds, roughly the size of a corn on the cob. Key developments during this period include:

• Increasingly coordinated movements and responses to external sounds.
• Sophisticated hearing capabilities, detecting internal and external sounds.
• Critical lung development with the start of surfactant production.
• Accumulation of fat under the skin for better temperature regulation.
• Rapid brain development with significant neuronal connections.

This period is crucial for your baby’s growth and development, laying the groundwork for future advancements in the subsequent months of your pregnancy week by week.

Month 7 (Weeks 25-28): Preparing for Birth

As you enter the seventh month of your pregnancy, your baby is making significant strides in development. This period is crucial for their preparation for life outside the womb.

Weeks 25-26: Skin and Lung Maturation

During weeks 25 and 26, your baby’s skin begins to look less wrinkled and more like a newborn’s due to the accumulation of body fat. Their lungs continue to mature, producing surfactant that will help them breathe after birth. The nervous system is also maturing rapidly, improving your baby’s coordination and responses to external stimuli.

Weeks 27-28: Eye Opening and Position Changes

In weeks 27 and 28, your baby can open their eyes and blink, with fully formed eyelashes. The production of melanin determines their eye color, although this may change after birth. Many babies start to move into a head-down position in preparation for birth around week 28.

By the end of the seventh month, your baby is about 14 to 15 inches long and weighs between 2 and 3 pounds. Their brain development is rapid, with emerging complex brain wave patterns that indicate periods of REM sleep.

Third Trimester: Weeks 28-40

The third trimester marks the final stage of your pregnancy journey. During this period, your baby undergoes significant growth and development, gaining weight and preparing for life outside the womb.

As you progress through weeks 28-40, your baby will continue to develop physically and cognitively. The third trimester is crucial for your baby’s brain development, fat accumulation, and overall preparation for birth.

Final Growth and Development

Your baby will grow from about 2-3 pounds to approximately 7-9 pounds during the third trimester. This weight gain is essential for regulating body temperature after birth. The weeks ahead will also see significant advancements in your baby’s organ systems and sensory development.

Your Body’s Preparation for Birth

As your body prepares for birth, you may experience hormonal changes that soften ligaments and joints, particularly in the pelvis. Your uterus will also practice contractions, known as Braxton Hicks. It’s essential to attend prenatal appointments every two weeks until week 36, and then weekly until delivery, to monitor your baby’s position and growth.

Month 8 (Weeks 29-32): Brain Development

The eighth month of pregnancy is crucial for your baby’s brain development, as billions of neurons form connections that will support learning and development after birth. During this period, your baby continues to mature and develop reserves of body fat.

Temperature Regulation

Between weeks 29 and 30, your baby gains the ability to regulate their own body temperature through improved nervous system function and increased fat stores. You may notice the kicks and jabs feel more like pokes as your baby gets cramped in the amniotic sac. By week 30, your baby can control its own body heat, and its brain is maturing and growing rapidly.

Organ Maturation

During weeks 31-32, your baby’s organs continue maturing, with the digestive system now capable of processing some nutrients and the immune system developing antibodies. Your baby can process more information and stimuli, and you can probably notice more distinct patterns in when it’s awake and when it’s asleep. By week 32, your baby’s skin isn’t translucent anymore, and most organs are well-formed and ready for birth, except for the lungs and brain.

As your baby grows, you’ll notice changes in their movements, such as more defined pushing, rolling, and hiccups. By the end of month 8, your baby measures about 16-18 inches long and weighs approximately 4-5 pounds. As one expert puts it,

“The third trimester is a period of significant growth and preparation for life outside the womb.”

Month 9 (Weeks 33-36): Final Touches

During the ninth month, your baby undergoes significant changes, refining their features and preparing for life outside the womb. This period is crucial for finalizing growth and brain development.

Bone Hardening

Between weeks 33 and 34, your baby’s bones continue to harden, except for the cranial bones, which remain soft to facilitate a smoother passage through the birth canal. This bone hardening is a significant milestone, as it prepares your baby for life outside the womb. Your baby’s immune system also strengthens during this time, thanks to the transfer of antibodies from your bloodstream, which will provide protection during the first few months after birth.

Losing Lanugo

In weeks 35 and 36, your baby starts to shed the lanugo, the fine hair that covered their skin since the second trimester. Although some babies may still be born with patches of lanugo, most lose it by the end of this period. The vernix coating on your baby’s skin becomes thicker, continuing to protect it from the amniotic fluid. Your baby’s brain continues to grow, although it will still be about two-thirds of its birth weight by week 36.

By the end of month 9, your baby is approximately 17-19 inches long and weighs around 5-6 pounds, roughly the size of a honeydew melon. Most babies move into a head-down position during this month, with their head settling into your pelvis, a process known as “lightening” or “dropping.”

Pregnancy Week-by-Week: What’s Happening Inside Your Body During Month 10 (Weeks 37-40)

You’re now in the last stretch of your pregnancy journey, and your baby is getting ready for birth. At this point, your baby is fully formed and just needs a bit more time to mature. You may feel very uncomfortable as your baby drops down into your pelvis and prepares for birth.

Weeks 37-38: Full Term Approaches

During weeks 37-38, your baby continues to gain weight at a rate of about half a pound per week. This weight gain is crucial for your baby’s temperature regulation and energy after birth. Your baby’s lungs and brain are still maturing, but their systems are fully developed.

Weeks 39-40: Ready for Birth

By weeks 39-40, your baby is considered full-term and ready to meet the world. Your baby’s organs are mature enough for life outside the womb, and they typically measure 18-20 inches long and weigh approximately 7-9 pounds. You should be prepared to contact your pregnancy care provider if you notice any signs of labor.

As your due date approaches, you may feel a mix of excitement and anticipation. Your baby’s movements may become less frequent but more pronounced as space becomes extremely limited.

Conclusion

Your pregnancy journey, spanning 40 weeks, is a testament to human development. Throughout this period, your body undergoes numerous changes as your baby grows.

Understanding week-by-week changes helps you connect with your developing baby and makes sense of your physical symptoms. Only about 5% of babies arrive on their due date.

Regular prenatal care, including ultrasounds, ensures your baby’s healthy development and prepares you for birth. As you approach your due date, trust that your body and baby are ready for birth after 40 weeks of growth.