Fetal/Baby Development in the First Trimester

The moment you find out you’re pregnant, everything changes. There’s a whirlwind of questions, information, and to-do lists inside your head.

You’ll probably start imagining your little ones, how they’re developing, and what they’ll be like once they arrive. How big are they now? What about now? When will you feel them kick? When can they hear your voice?

The First Trimester

Fetal development starts soon after conception and usually follows a predictable, steady course. Keep reading to learn more about everything your baby’s up to in the first trimester! 

Week One & Two: Your Last Menstrual Period and Conception

Although this might come as a surprise, you’re not actually pregnant for the first week or two of pregnancy. Conception usually occurs about two weeks after your last menstrual period begins.

That means, to calculate your estimated due date, your healthcare provider will count ahead 40 weeks from the start of your last period. So, your period is counted as part of your pregnancy—even though you weren’t pregnant then. 

Week Three: Fertilization and Zygote Development

The sperm and egg come together in one of your fallopian tubes to form one entity called a zygote. If more than one egg is released and fertilized, or if the fertilized egg splits in two, you might have multiple zygotes—twins!

Usually, the zygote has 46 chromosomes, 23 from the biological mother and 23 from the biological father. These chromosomes help determine things like your baby’s sex and physical traits.

Soon after fertilization, the zygote travels down the fallopian tube toward the uterus. At the same time, it’ll start dividing to form a cluster of cells resembling a tiny raspberry, called a morula. 

Week Four: Implantation and Embryonic Formation

This ball of cells will start rapidly dividing; at this point, it’s known as a blastocyst. It starts to burrow into the uterine lining, which is called implantation. Inside the blastocyst, the inner group of cells will become the embryo!

As the embryo begins implantation, its outer layer will give rise to these essential structures formed to nourish and protect your baby throughout your pregnancy:

  • The Umbilical Cord: The rope-like umbilical cord is made up of a vein and two arteries and provides your baby with oxygen while exchanging waste and nutrients from your body in the placenta.
  • The Placenta: The placenta is a unique organ that attaches to the uterine wall; it only grows and exists in the body when a woman is expecting a baby. At the point of attachment, fetal blood vessels in the umbilical cord exchange waste and nutrients through the mother’s blood supply.
    A thin membrane that covers the fetal side of the placenta—the amnion—prevents fetal blood cells from crossing into the mother’s bloodstream.
  • The Amniotic Sac: The amnion and fetus are enclosed in the amniotic sac throughout embryonic and fetal development. The sac fills with amniotic fluid, which regulates temperature and protects your baby while you anticipate its arrival.

Week Five: Hormones, Placenta Growth, and Embryo Development

This is your fifth week of pregnancy, but remember—it’s only the third week after conception. The levels of HCG hormone produced by the blastocyst quickly increase. This signals your ovaries to stop releasing eggs and produce more estrogen and progesterone. 

These increased levels of these hormones stop your menstrual period, which is often the first sign of pregnancy! It also fuels the growth of the placenta, amniotic sac, and umbilical cord.

Development of Embryonic Layers

The embryo also now has three layers:

  • The top layer, the ectoderm, will eventually become your baby’s outermost layer of skin, central and peripheral nervous systems, eyes, and inner ears. 
  • The middle layer, the mesoderm, will become the heart and primitive circulatory systems. The mesoderm also fosters the future development of your baby’s bones, reproductive system, parts of the urinary system, and ligaments.
  • The innermost layer, the endoderm, will become your baby’s lungs and intestines. 

Week Six: Development of Nervous System and Major Organ Systems

Your Baby’s Brain, Spinal Cord, and Neural Tube

Your baby’s development will kick it up a notch this week! The neural tube along your baby’s neck is almost closed just four weeks after conception. Their brain and spinal cord will develop from the neural tube, initiating electrical activity in the nervous system.

Your Baby’s Heart, Major Organ Systems, and Limbs

Your baby’s heartbeat will start, and its sensory organs begin to form along with the digestive tract; every other major organ system has begun to form or resume development.

Small buds appear that will eventually become your baby’s arms and legs, and their cartilage frame becomes bone. Your baby’s body begins to form a C-shape at this point in your pregnancy. 

Week Seven: Your Baby’s Head Develops Structure

Seven weeks into your pregnancy, five weeks after conception, your baby’s brain and face are developing. Little sunken areas in your baby’s face are visible, which will eventually become their nostrils.

Their retinas are just beginning to form. Their legs follow close behind their arms—little buds where their legs will be are visible now. The little arm buds are now resembling paddles.

Week Eight: Your Baby Begins to Develop Facial Features and Fingers

In the eighth week of pregnancy, your baby’s nose forms, as well as their upper lip! Your precious baby’s lower limb buds look like paddles at this point, and their fingers are beginning to take shape. Your baby’s ears and eyes are apparent now, and their posture is starting to straighten out. 

By the end of this week, your baby is somewhere around a half-inch long—about half the diameter of a United States quarter!

Week Nine: Your Baby’s Toes and Eyelids Form

In your ninth week of pregnancy, seven weeks after conception, your baby’s arms grow, and his or her elbows appear. This point of fetal development includes the first stage of their developing eyelids, as well as their toes.

Your baby’s head is pretty large at this point, but they don’t quite have a chin yet. By the end of this week, your baby is a little less than ¾ inch long—the diameter of a U.S. penny. 

Week Ten: Final Embryonic Stage

By your tenth week of pregnancy, eight weeks after conception, your baby’s head has gotten much rounder. In this stage of fetal development, your baby’s elbows bend, but you won’t feel any tiny elbow jabs until you’re closer to the third trimester.

Their toes and fingers lose their webbing and start to get longer. Eyelids and the future shell-shaped parts of their external ears are continuing to develop. The umbilical cord is now clearly visible.

Week Eleven: Reproductive Organs Begin to Form

Your baby’s head makes up about half its length right now—but don’t worry, their body will soon catch up! Your baby is now officially described as a fetus at this point.

Their face is broad at this stage with their eyes widely separated and their eyelids fused, and your baby’s ears develop, set low on their head. Buds for their future teeth appear at this point. Red blood cells are also beginning to form in your baby’s liver. 

By the end of this week, your baby’s external genitalia (the penis or clitoris and labia majora) will start developing! Your baby’s body is now close to the length of the short side of a credit card, at around two inches long, and weighs approximately 1/3 ounce.

Even though your baby’s genitals develop by this time, it’s highly unlikely that your OB-GYN or midwife could find out your baby’s gender using a sonogram in the first trimester. There are plenty of theories, methods, and some myths on how to determine whether your baby is a boy or girl this early, but your best bet is to wait until you’ve reached 13 weeks.

Week Twelve: Final Stage of the First Trimester

In the twelfth week of pregnancy, your baby’s fingernails form, and their circulatory and urinary systems are fully functioning! Their face has now taken on a more developed profile, and their intestines are now developed in their abdomen; the liver produces bile—an integral component of the digestive process.

Your baby is now around the length of the short side of a U.S. dollar bill at about 2 ½ inches long and weighs roughly 1/2 ounce.

Signs Your Pregnancy Is Going Well in the First Trimester

Healthy progression in the stages of fetal development typically goes hand-in-hand with a number of signs and symptoms in expecting mothers’ bodies that can let you know whether or not things are going well.

During your pregnancy, your body is sure to go through numerous changes; but some women can experience a multitude of these symptoms, while others may have little to none at all:

  • As we mentioned before, your monthly menstruation is likely to cease around the fifth week of pregnancy for the rest of your pregnancy. On the other hand, if you notice any bleeding or spotting, seek advice from your physician as soon as you can.
    An excess or increase in clear vaginal discharge, however, is actually a good sign that your estrogen levels are where they should be.
  • In the early pregnancy stages of the first trimester, many mothers begin to experience morning sickness. Nausea and vomiting can occur while your body adjusts to the boost of hormone levels in your body, and it’s unlikely that morning sickness could disturb your or your baby’s health.
    Despite what the term “morning sickness” implies, many pregnant women can encounter these waves of nausea at any time of day, not just in the morning.
  • As the levels of estrogen and progesterone in your body increase, you should expect to see changes in your breasts such as swelling, darkened areolas, more prominent veins, tenderness, and enlarged mammary glands.
    These changes are prepping your body for nursing, and maternity bras are an excellent investment for support, comfort, and convenient breastfeeding access.
  • Pressure on your bladder, rectum, and intestines from your growing uterus and baby can cause more frequent urination and constipation, especially in the last few weeks of your third trimester when he or she gets ready to move down towards the birth canal.
    Your body’s increasing progesterone levels are also a likely cause of constipation and other symptoms of delayed digestion. The hormone slows the rate of muscle contractions which regulate typical digestive tract function.
  • You may want to create a maternity-friendly wardrobe now, as your beautiful growing belly and breasts will soon feel a little constricted in your old clothes.
    Of course, this is totally natural, and options for maternity clothing make it much easier to get dressed and are often quite comfortable—stylish, even.
  • You’re likely to become fatigued—physically and emotionally. You may even have mood swings similar to the feelings you probably experienced during your previous menstrual cycles.
  • Since the boost of hormones during pregnancy can heighten the full spectrum of your emotions, you may see that they spark joy and help you to feel relaxed while you’re expecting.

Some of these or other symptoms may not be something you’re looking forward to, but they are typically no cause for alarm, as they’re signs that your pregnancy is progressing at a normal, healthy rate! If you think you might experience something out of the ordinary, go ahead and give your doctor or midwife a call.

In the Second Trimester

Stay tuned for our complete breakdown of the second and third trimesters! Until then, check out our blog for more information.

Our maternal medical care team in the Women and Children’s Pavilion at RMC in Anniston welcomes more than 2,000 newborns each year. We have a team of specially trained OB Nurses, board-certified physicians, and top-of-the-line labor, delivery, and recovery care systems.

For all the pregnancy advice and support you need, contact Regional Medical Center today!