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Fetal development – week by week

fetal development week by week

Table of Contents

How’s My Baby Growing? – A fetal development week-by-week review 

For a lot of women, pregnancy and childbirth will be a defining moment in their lives. Some people who have been through this might even say that it was one of their most rewarding experiences. As a mum to be, I can tell that you’re curious – believe me, it’s normal. You’ve bought the books, you’ve watched the videos; you’re even reading this article right now. I get it – you can’t wait to meet your little one and you want to be ready when they arrive. Well, it’s a great thing you’re here! In this post, we will be discussing the different stages of fetal development on a weekly basis.

Prenatal Development

Prenatal development describes the entire baby development process, from the menstrual age, to fertilization, up until they’re born. Prenatal development occurs in three stages: The germinal stage, the embryonic stage, and the fetal stage.

The Germinal Stage

Fetal development begins with the germinal stage. That counts from the day of your last menstrual period and lasts for about three weeks. In case you didn’t already know, conception is not the beginning of your pregnancy; all the work your body has done to make fertilization possible has to count for something. While pregnancies are usually reported to last about forty weeks, in reality you’re only pregnant for thirty-eight of those weeks. This is because your pregnancy starts to count from the first day of your last menstrual period. Now let’s look at this more closely.

Week 1: You see your period. This is your body shedding off the lining of the uterus and preparing to begin a new menstrual cycle. By the end of this week, your period is coming to an end and your body starts to produce more hormones.

Week 2: Your estrogen and progesterone levels are all the way up and this prompts your uterine wall to thicken. While this is happening, your ova are “ripening” in fluid-filled sacs commonly referred to as follicles. Around the time week two is coming to an end, your body will release another hormone (lutrophin). This hormone causes the follicles to rupture, thus releasing the ovum/ova (or egg/eggs) into the fallopian tube to make its way to the uterus. The process we have just described is what you have always known as ovulation. (It is important to note that, although ovulation commonly occurs in the middle of the menstrual cycle, this is not always the case. Ovulation can occur from any time between nine to twenty-one days from the first day of your last period).

The egg usually takes between one to two days to travel from the ovaries to the uterus. While it’s on the way to the uterus, if one of the numerous sperm cells released during ejaculation is able to penetrate the egg, then fertilization occurs. Within the next 48 hours, the sperm cell fuses with the egg and their DNAs are combined to form a zygote

By the end of week 2, the zygote continues its journey from the fallopian tube to the uterus. While it’s on its way, it will continue to divide into hundreds of identical cells. In about two days, the zygote (which is now a blastocyst) will attach itself to your uterine wall. The end of week two also marks the end of the germinal stage.

The Embryonic Stage

The embryonic stage lasts for about eight weeks after fertilization. This stage is very important in the development of brain function. It is therefore important to get your nutrition right in this stage.

Week 3: It is from this week that you are likely to get a positive pregnancy test. The blastocyst will continue to divide and burrow further into your uterine wall. The inner cells of the blastocyst will grow into the embryo while the outer cells will grow into the embryo. The outer cells will begin to produce the human chorionic gonadotropin (hCG), also known as the pregnancy hormone. The pregnancy hormone is what is detected when you pee on a stick. This hormone also tells your ovaries to stop releasing eggs and progesterone.

Week 4: Now your little embryo is about the size of a poppy seed. The inner cells will further divide into three more layers. The first layer will develop into the digestive and respiratory system (the stomach, lungs, bladder, etc.). The middle layer will develop into the circulatory, muscular, and skeletal system, while the outer layer develops into the nervous system, the eyes, tooth, skin, and nails. Before the placenta is fully formed, the embryo will receive nutrients and relieve waste through a yolk sac. Later on in your pregnancy, the placenta will form and take over from the yolk sac.

Week 5: Around this time your baby’s heart and nervous system have started to develop. You should start taking folic acid at this stage because it can prevent a condition called spina bifida in your baby. Spina bifida is a condition in which your baby’s spine and spinal cord are underdeveloped, such that there’s a gap in the spine.

Week 6: At this stage, the embryo looks like it has a tail. Your baby’s heart is still developing, while their little arms and legs have also started to grow out as buds.

Week 7: Now your baby is about 10mm long and the brain continues to develop rapidly. Their inner ears and eyes have also started to develop. The limb buds will get longer and start to form cartilage which will develop into the limb bones.

Week 8: Now your baby is about the size of a kidney bean and they weigh about 1.13 grams. Still tiny, right? I know. Although the lower limbs are getting longer, they’re not yet distinct. Your baby now has toes and fingers and their respiratory system continues to develop. The 8th week is the end of the embryonic stage. After this week, your pregnancy is moving into the longest and final stage; the fetal development stage.

The Fetal Stage

The fetal stage is the longest and probably the most dramatic stage of prenatal development. When fetal development begins, cell differentiation is complete and the baby is no longer referred to as an embryo, but a fetus. We will now discuss this more extensively by looking at development by week.

Week 9: By now your baby has almost all their essential organs and they look like a really tiny human. All the chambers of your baby’s heart have formed and their tiny baby teeth started to grow. By week 9, the placenta is almost fully developed and ready to take over from the yolk sac.

Week 10: Your baby is now about 3.5 mm and their outer ears have started to develop. The canals between the inner and outer ears are also forming. Jaw bones are developing and your baby’s heart is fully pumped and beating. Although the eyelids are starting to form too, they are sealed shut until about the 27th week. Around this time, the placenta should be fully-formed and the yolk sac shrinks until it is fully resolved. In some people, the placenta may not take over even up to the 20th week.

Week 11: Your baby is now the size of a fig. The bones of their face are formed and the outer ears are starting to look more like ears now. Their fingers and toes are separating and their external genitalia will start to develop around this period.

Week 12: You are now coming to the end of the third month and your baby is about the size of a lime. It is now 57mm long and weighs about 14 grams. Your baby has started to develop reflexes and they can now curl their toes. The intestines are fully formed and enclosed in the abdominal cavity now. The external sex organs should be well developed alongside all other organs. Now all your baby has to do is grow.

Week 12 is the end of the first trimester of pregnancy. The most critical period of your baby’s development has been completed and you are less likely to miscarry after this period.

Week 13-15:  This is the beginning of the second trimester. Your baby’s heartbeat can now be heard. Their fingers and toes are well-defined and their bones have become denser. Around this period, eyebrows, eyelashes, fingernails, and toenails are formed. Now your baby can stretch, yawn, and even make faces. By the end of the 15th week they’re about 152 mm long and 113 grams heavy. Their external genitalia is fully formed and you can now tell the sex of your baby via an ultrasound scan.

Week 16-19: If you haven’t already told them yet, I’m sure your friends and family can already tell you’re pregnant. Your baby is now developing muscles and you may feel them move as they exercise these muscles. Your baby is covered with vernix caseosa, which is a white cheesy substance that protects them from elongated exposure to amniotic fluid. Don’t fret, they’ll shed this coating just before they’re born. By now, your baby should be about 25.4 cm long and about 250 grams heavy.

Week 20-23: You’re coming to the end of your second trimester. Congrats! You’re almost there. Your baby now has fingerprints and you can see their veins through their translucent skin. Those little movements from earlier have become full-fledged kicks.

Week 24-29: This is the beginning of the third and final trimester. Now that their hearing is fully developed, you may want to sing to them every now and then. They can also respond to stimuli, so you might notice them changing position frequently in response to light or sound. It is around this period that the amniotic fluid starts to diminish. Their eyelids also open up around this period. By the end of the 29th week, your baby should weigh anywhere between 900g to 1.5kg.

Week 30-35: Your baby starts to develop reserves of body fat. They can see, they can ear, and they’re kicking more now. Their brain and all internal organs are almost fully developed. Their lungs, however, still need some time to mature fully. Now you baby should be weighing in at about 2kg.

Week 35-40: This is it! You made it this far, your baby is almost here. The lungs develop fully and their reflexes are better coordinated so they can close and open their eyes, turn their heads, and even grasp items. As you approach your due date, you may notice that your baby has started to move less. Don’t be quick to panic, there’s just less space for that now! As you approach week 40, your baby starts to position itself for delivery. It’s head is facing downward and it drops into your pelvis. Now your baby is about 3kg and ready to be born!

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