Development calendar: Pregnancy

An average pregnancy lasts 40 weeks or 280 days, but most babies are born between 38 to 42 weeks after the last menstrual period. Every pregnancy is made up of three trimesters. During each trimester, your baby is going to be developing each week to help prepare them for life outside the womb. Here is a look at what is going to happen each week of your pregnancy.

Week 1:
This is actually when you start your menstrual period. The baby has not even been conceived yet, but because your due date is calculated from the first day of your last period it has to be counted.

Week 2:
Near the end of this week is when the fertilization of your egg is going to take place. From the moment that your egg is fertilized the sex of your baby is going to be determined.

Week 3:
This is when the fertilized egg begins the process of cell division. As the cell moves through the fallopian tube and into the uterus, it forms a group of cells that is called a morula. The morula then turns into the blastocyst, which is when it will attach itself to the lining of the uterus.

Week 4:
At this stage, your fertilized egg is now referred to as an embryo. It now consists of two layers of cells, the epiblast, and the hypoblast. These two layers of cells will develop into your baby's organs and body parts. This is also when the amnion, which fills with amniotic fluid to protect your baby, and the yolk sac, which produces blood to nourish the embryo until the placenta takes over, is developed.

Week 5:
This is when the embryo starts to form a distinct shape, although it does not yet start to look like a baby. During this week, the placenta is developed to provide the baby with nourishment.

Week 6:
Your baby's brain and nervous system are developing at a rapid pace. The optic vesicles and the passageways for the inner ear are also developing. Your baby's heart will begin beating at this point, and in some cases, the heartbeat can be detected on an ultrasound examination. The digestive and respiratory systems are starting to develop.

Week 7:
Your baby's umbilical cord has formed, which is a vital part of the baby's formation. The umbilical cord is what brings the baby oxygen and nourishment, as well as disposing of the baby's wastes. The digestive tract and lungs are continuing to form. Your baby's face is also starting to take shape.

Week 8:
The fingers and toes begin to form, and the arms can flex at the elbows and wrists. The eyes are becoming more pronounced because they have started to develop color in the retina. The genitals are beginning to form this week, however they are not big enough to show you if it is a boy or a girl yet.

Week 9:
Your baby's head has been growing, and is quite large when compared to the rest of the baby's body. The tip of the baby's nose can be seen in profile, and the skin above the eyes has begun to shape into eyelids. At this point, your baby weighs around .1 ounces and measures about .6 to .7 inches long.

Week 10:
All of your baby's vital organs have been formed, and they are starting to work together. The fingers and toes are beginning to separate and the tail from the spinal cord has disappeared entirely. This is also the end of the embryonic period because the embryo has now taken on a distinctly human appearance.

Week 11:
Your baby is now considered a fetus. Facial development is continuing, the ears are moving toward their final position on the sides of your baby's head. The head is about half the length of the body, so it is still large compared to the rest of your baby. Reproductive organs are developing rapidly, but the external genitals will not be any different until the end of week 14.

Week 12:
The brain is continuing to develop. Fingernails and toenails start to form this week, as well as the vocal cords. Because your baby's kidneys are now functioning the baby can pass urine, the intestines can now fully fit inside the abdomen. This week also marks the last week of your first trimester

Week 13:
The placenta is fully developed so it can provide your baby with oxygen, nutrients, and waste disposal. The eyelids have fused together to protect the eyes as they develop. The sucking muscles have started to develop, so your baby might even put their thumb in their mouths this week.

Week 14:
The genitals have fully developed, though they might be hard to spot on an ultrasound. Fine hairs called lanugos has begun to cover your baby, they will keep this hair until just before delivery. At this point, your baby weighs about 1.6 ounces and is about 3.5 inches long.

Week 15:
Hair growth continues on the eyebrows and the head. The skeletal system continues to develop; muscle development also continues.

Week 16:
Your baby weighs about 3.9 ounces and measures about 4.7 inches. Your baby can hold its head erect and has fully developed facial muscles that allow for a variety of facial expressions.

Week 17:
Your baby measures about 5.1 inches and weighs about 4.9 ounces, which is still rather small. While your baby continues to grow so does the placenta so that it can accommodate your baby as it grows.

Week 18:
The ears have moved to their final position and stand out from the head. The bones of the middle ear and the nerve endings from the brain are developing so that your baby can hear. The eyes are developing; they face forward rather than to the sides. The retinas can also detect a beam of light if it is held to your abdomen. The bones of your baby begin to harden this week, starting with the clavicles and the legs.

Week 19:
Baby is now covered with a waxy substance called vernix caseosa, which protects the baby's skin from being damaged. Brown fat is beginning to develop at this stage to help keep your baby warm after birth.

Week 20:
This is the halfway point of your pregnancy. Your baby weighs about 11 ounces and measures about 6.3 inches. Hair and nail growth continue, as well as the skin thickening and developing layers.

Week 21:
The amniotic fluid that was first used to cushion and support your baby now provides your baby with sugars, which is absorbed when they swallow the amniotic fluid. The bone marrow, along with the liver and spleen are producing blood cells.

Week 22:
Taste buds have started to form on the tongue. The brain and nerve endings are formed enough so that your baby can feel touch. The reproduction system is continuing to develop, with boys, the testes have begun to descend from the abdomen and in girls the uterus, and ovaries are in place.

Week 23:
At this point, you will begin to feel forceful movements because of your baby's daily workout routine. At this point, your baby weighs about 1 pound. If you went into preterm labor at this point with expert medical care your baby could survive, however they might have mild to severe disabilities.

Week 24:
Lungs are developing the ability to produce surfactant, which is a substance that keeps the air sacs in our lungs from collapsing and sticking together when we exhale. The inner ear is now fully developed, so your baby can tell when they are upside down or right side up while floating in the amniotic fluid.

Week 25:
You will begin to notice that your baby has resting and alert periods. You will also notice an increase of fetal movements, when you are sedentary. Your baby's hearing is fully developed so they can hear your voice.

Week 26:
Your baby's eyes will begin to open and blink. The eyelashes are also growing in, as well as more hair on the head. At this point, your baby weighs a little less than 2 pounds. This week marks the end of the second trimester.

Week 27:
Your baby looks similar to what they are going to look like at birth, except they are thinner and smaller. Your baby might now be able to recognize other important voices, although the sounds are going to be muffled due to the ears being covered with vernix.

Week 28:
Your baby now weighs about 2 pounds, 2 ounces and is about 10 inches long. The folds and grooves of your baby's brain continue to develop and expand. Your baby also continues to add layers of fat and is continuing hair growth.

Week 29:
If at this point you notice a decrease in movements you will want to so a fetal kick count. Your baby should move at least 10 times in one hour. If the baby is moving, less you need to talk to your doctor during your next visit.

Week 30:
Your baby now weighs about 3 pounds and is about 10.8 inches long. Your baby is also mimicking breathing movements by repeatedly moving their diaphragm. Your baby can also get the hiccups, which you can feel as rhythmic twitches in your uterus.

Week 31:
The levels of your amniotic fluid will be measured as a routine part of your ultrasound to check for too much or too little fluid, which can indicate a problem. Your baby is swallowing enough amniotic fluid to produce several cups of urine each day.

Week 32:
This is when the final touches are starting to take place. The lanugos hair that has covered your baby since the second trimester is starting to fall off, however some might remain on the shoulders and back at birth. Your baby weighs 4 pounds and measures 11.4 inches. If delivered now the baby has an excellent chance of surviving outside of the womb.

Week 33:
The neurons in your baby's brain are helping them to learn about the in-utero environment. Your baby can listen, feel, and even see somewhat. Your baby's lungs are almost fully developed and fat is continuing to be deposited on your baby's body.

Week 34:
The vernix coating is becoming thicker, while the lanugos hair is almost entirely gone. Your baby will now be in the position for delivery, hopefully headfirst because babies who are feet first might need to be delivered by a cesarean section.

Week 35:
This week begins a rapid period of weight gain, even though by now your baby weighs about 5 pounds, 5 ounces. Your baby will gain about 8 to 12 ounces each week. Due to the lack of space in your uterus because of how big your baby is fetal movements might decrease, but they might also be more forceful and stronger.

Week 36:
Your baby now has fat on their cheeks and their sucking muscles are fully developed, which helps contribute to your baby's full face. The bones in your baby's skull can move relative to one another and overlap each other, which help the baby, pass through the birth canal.

Week 37:
Your baby is now considered full term. Your baby will continue to grow, in length and in weight. Your baby has enough coordination to grasp with their fingers. In addition, if shown a bright light they might turn toward it in your uterus.

Week 38:
Fat is still accumulating, but growth has slowed down. In fact, you might notice that your weight gain has decreased or stopped altogether. If the baby is a boy the testicles will have descended into the scrotum, if it is a girl the labia is now fully developed.

Week 39:
Most of the vernix that covered your baby's skin has disappeared, as well as the lanugos. Your body has been supplying your baby with antibodies through the placenta so that they can help the baby's immune system fight infections for the first six to twelve months of life.

Week 40:
Only 5% of babies are born on this date, the other 95% can be born two weeks before or two weeks after. At forty weeks, your baby weighs about 7 pounds, 4 ounces and is about 20 inches long. If born after 40 weeks they can be bigger because they will continue to grow inside the womb.