Development of the baby weeks 1 to 20

Below you can read all the information about your baby, your own development and other fun facts per week of your pregnancy. The information from various websites has been combined so that you have a complete picture of what you will experience these weeks.

Week 1

0 mm and 0 grams
Strange but true, in this week you are not actually pregnant yet, because fertilization still has to take place. If you have a regular cycle, you will ovulate next week. To accurately measure when your baby is due, the first day of your last menstrual period is used. The growth of a fertilized egg into a complete baby takes 38 weeks. By taking the first day of your last period and adding 40 weeks, you can calculate when your due date will be. From the moment you want to become pregnant, it is wise to avoid a number of things, namely smoking and drinking alcohol. Tell your doctor that you want to become pregnant if you are taking medication. It is smart to take folic acid from the moment you stop taking the pill, as this reduces the risk of spina bifida and open skull in the baby. Folic acid is available at the drugstore or pharmacy.

Week 2

0 mm and 0 grams
You are not yet pregnant in week two. The egg is maturing and ready for fertilization when it emerges from the ovary. We call this ovulation. The egg cell ‘waits’ in the fallopian tube for fertilization by the sperm cell. However, an egg cell only lives for 24 hours, during which fertilization must take place. In other words, sex must take place during that 24 hours. Sperm cells live longer, namely 48 hours. So if you have had sex 1 or 2 days before ovulation, fertilization can still take place. You can calculate your ovulation, it is approximately the 40th day of a regular cycle. When it is precise, you can measure it with a thermometer, after ovulation your temperature rises to a lower level. It is even easier to buy a test, because it calculates it for you. These are available at a drugstore or pharmacy. The first two weeks do not count for the ultrasound scan, but they are necessary to calculate when your baby will be born. Although only four percent of all children are born on their due date.

Week 3

0.1 mm and 0 grams
You are not overdue, but you may already feel a little different. Read here about what the first symptoms of pregnancy are. The sperm cell and egg cell now fuse together. After fertilization, the egg cell begins to divide into cells at a rapid pace. In the meantime, your uterus is preparing under the influence of the hormones estrogen and progesterone. When the egg reaches the uterus, it is already ready. It is now only a clump of cells, but everything is already determined (hair color, eye color, gender and even the character). Implantation will take place approximately next week.

Week 4

1 mm and 0 gram
The fertilized egg is less than 1 millimeter. You may already notice that you are pregnant, for example due to tiredness or nausea. But you can only be sure with a pregnancy test. The cells continue to multiply. The outer layer of cells form the basis for the placenta. The placenta will ensure that nutrients pass through your blood into your baby’s blood. The inner cells (embryonic node) grow into different body parts. There is also a third group of cells, these form the membranes. The cervix is closed with a mucus plug and blood vessels emerge under the mucous membrane, so that nutrients can easily reach the embryo. At the end of this week, the embryo has implanted in the uterine wall. You may lose some blood, implantation bleeding.

Week 5

2 mm and 0 grams
The fertilized egg has already doubled in size in this short time. It is now two millimeters in size! Between this and next week, a start will be made on the central nervous system, or at least its construction, but a start will also be made on the brain and the heart. It is possible that the fertilized egg splits in two, both forming a separate fruit and subsequently becoming identical twins. These babies are always of the same sex, look very much alike and have the same hereditary characteristics. If instead of one egg, two eggs mature, release at the same time and are both fertilized, they become fraternal twins. These babies can be of different genders and look completely different.

If you know you are pregnant, you can look for a midwife nearby. Or you can search for your choice on the internet, ask your doctor or acquaintances for advice. You should give up a number of foods in the coming period, namely raw milk cheeses (au lait cru), raw meat and do not eat too much organ meat (excess of vitamin A). Click here for more information about what to eat/drink during pregnancy. Large temperature changes are also not good for the development of the baby’s organs, so a visit to the sauna is also out of the question.

Week 6

6 mm and 0 grams
The fertilized egg increasingly resembles a grain of rice and a shrimp. Four bumps are visible, which grow into arms and legs. In this week the foundation is laid for the chest, stomach, intestines, head and lungs. A start is also made on the network of blood vessels. At the end of this week, the heart will contract and the blood will pump through the baby’s body, special, right? And if an ultrasound were to be made now, there would already be a very vague presence of a spine. You probably already feel more that you are pregnant, perhaps due to tiredness and nausea, the typical initial ailments. Do you also have to urinate more than normal? This is due to hormones in the beginning of pregnancy and due to the uterus pressing on the bladder in the last phase of pregnancy. If you have a fever or high temperature during this period, consult your doctor. Temperature fluctuations are not good for the baby.

Week 7

10 mm / 1 cm and 1 gram
The small bumps on the body start to grow into arms and legs. Toes and fingers will also start to take shape, but these resemble swimming fins. The head is also taking shape, there are already eyes, a nose, nostrils and a mouth. The heart is already completely finished and has four chambers. If you were to have an ultrasound now, you would already be able to see the heart beating. But you would only have this ultrasound for medical reasons, such as blood loss (which does not necessarily mean anything serious, perhaps it is implantation bleeding or a burst blood vessel in the uterus). The spinal cord and brain are almost completely finished. The head is very large in relation to the rest of the body and will remain that way for some time. Your baby is now growing about 1 millimeter (mm) per day.

Week 8

20 mm / 2 cm and 2 grams
The head is starting to look more and more like a real face. The skin is paper thin, the blood vessels shine through. Until now the baby was a week shape, but now the skeleton is starting to emerge. It does not yet consist of bone, but of cartilage. Hips, shoulders, elbows and knees are formed during this week. Your baby will become more active and will move its arms and legs, although you cannot feel it yet. It is now time to call the midwife for an appointment in the 12th week. The midwife will schedule extra time for you to ask you about any previous pregnancies, medication use and hereditary diseases. Because these questions also apply to your partner, it is useful to go to the appointment together. Your body will also be examined and you will have to have your blood taken.

Week 9

30 mm / 3 cm and 3 grams
Your baby is finished. All organs and body parts are present, but of course everything is far from developed. The arms and legs, which are the size of a pea, start moving with small jerks. So you still don’t feel anything. The hands already have fingers and a thumb. The child also already has knees, ankles and toes. At this moment we no longer talk about an embryo, but about a fetus. That is what the child will be called until birth. There is a good chance that you are very tired during this period. Having trouble getting up in the morning and wanting to go to bed at 9pm in the evening. You may not be very happy with the pregnancy because of this. Don’t feel guilty about this, give in to it and go to sleep. After 12 weeks you will have a little more energy again and you can still do all the chores that could have been postponed.

Week 10

45 mm / 4.5 cm and 5 grams
Your baby’s head is still very large in relation to the rest of his body. The ears are ready on the inside and the auricle is starting to grow. The mouth contains an upper and a lower jaw, which contain 20 bumps, which grow into baby teeth after birth. From now on, the genitals and urinary system begin to form. In order to grow properly, the baby feeds through the umbilical cord. The umbilical cord contains three blood vessels that are connected to the placenta. There is a thick protective layer around the umbilical cord, which ensures that the blood vessels cannot become closed. Not even as if the baby were to ‘swim’ into a knot. You probably have quite a lot to deal with now, and you realize that you have to arrange quite a lot before the baby arrives. Don’t worry too much, because it seems like a lot, but you still have months to arrange everything. You can make a list and check something off every now and then. But relaxing and enjoying are much more important in this phase.

Week 11

55 mm / 5.5 cm and 10 grams
The baby has doubled in weight this week. And the baby has also grown 1 cm, this spurt continues until the 16th week. The internal organs are almost ready and starting to work and now it’s mainly about growing and getting stronger. The skeleton still consists entirely of cartilage. The head is becoming a bit rounder and the eyes are increasingly in the right place (they used to be more on the side of the head). They are still completely covered with eyelid. Your baby starts thumbing, yawning and stretching. This week the genitals also become visible. Hormones affect your skin, its condition can suddenly turn 180 degrees. If you have always had clear skin, it may now be covered with pimples, but the opposite is also possible.

Week 12

65 mm / 6.5 cm and 20 grams
The exciting first three months are behind you. Your baby’s face is already starting to take on more human shapes. There is a good chance that it is thumbing or trying to grab with its hands. The heart beats very quickly, with more than 160 beats per minute. The midwife will try to listen to the heartbeat with the doptone, or perhaps during an ultrasound. Every now and then the baby stretches and yawns unabashedly, but unfortunately you can’t see anything unless you have that exact ultrasound at that moment!

Week 13

75 mm / 7.5 cm and 30 grams
Your baby is now big and strong enough to push up your uterus. The top of the uterus is called the fundus. The distance between your pubic bone and fundus is used by the midwife to see whether your baby is growing properly. During this week the fundus is still at the level of your pubic bone. The amount of amniotic fluid in your uterus continues to increase. It is of vital importance for the baby, it can move freely, it serves as a cushion, provides constant warmth and teaches the baby to swallow and drink. The baby then pees out the amniotic fluid. Amniotic fluid is replaced throughout the day, which also removes waste products. Because the baby is now growing, you may also start to get a belly. If you had a belly before or you could no longer close your pants, it was caused by hormones or your intestines. One will get a belly quickly, the other not at all, this says nothing about the growth of the baby. Until 16 weeks, every baby grows the same in the womb, after which individual differences emerge. So as long as the midwife says the baby is growing well, you have nothing to worry about. Belly or no belly.

Week 14

85 mm / 8.5 cm and 60 grams
The swimming fins have disappeared and your baby can move his fingers and toes freely. It’s a hectic feeling in your stomach, even though you haven’t noticed anything yet. The baby is swimming nicely from left to right. And this is good for muscle development. Because he swallows amniotic fluid, your baby regularly gets hiccups. He will also choke every now and then. He’s already urinating a little, which means his kidneys are starting to work. The baby also develops downy hairs all over the body. And they remain in place until approximately the 37th week. They probably serve to keep the sebum (produced by the sebaceous glands, called vernix) in place to protect the baby against amniotic fluid and possible infections. If the baby arrives earlier than 40 weeks, it may still have downy hairs on its shoulders, back and face. Because the uterus continues to grow, your organs are now under slight pressure. Your entire body can now become slightly rounder, giving you a softer appearance. How much weight you will gain is still the question, on average people gain about 12.5-15 kilos. This is divided as follows: 1) 3400 grams of baby, 2) 1000 grams of uterus, 3) 800 grams of breasts, 4) 1000 grams of blood, 5) 3400 grams of extra fat and 6) 1500 grams of extra fluid.

Week 15

110 mm / 11 cm and 90 grams
Your baby is growing fast and puts all his energy into becoming bigger and stronger. He is now also growing eyelashes and his hair has become a lot thicker. He can also make funny faces: round his cheeks or furrow his eyebrows. The baby can open its mouth and press its lips together, it is not yet real sucking, but it is starting to look like it. He already weighs about an ounce. Your breasts are probably no longer as sensitive as they were in the beginning, but they are still large. And in the coming months they will get a little bigger, as the mammary glands prepare for breastfeeding. The amount of blood in your body also increases, this is necessary to help the uterus and placenta with their growth. Because you have 1.5 liters (on top of the 5 liters you already had) more blood in your body, your heart also has to pump harder to circulate it. As a result, the heart now beats an extra 15-20 times per minute.

Week 16

150 mm / 15 cm and 120 grams
He kicks his feet and waves his hands. His fingers and toes are also starting to grow tiny nails. It is already starting to develop a unique fingerprint, one that it will keep for the rest of its life. Although the eyes are still closed, it can distinguish light from dark through the eyelids. the eyes are now also framed by tiny eyelashes and eyebrows. You will probably go to the midwife for the second time this week. You will have approximately 13 check-ups until delivery. In the period up to week 24 you will visit the midwife once every 4 weeks, until week 30 once every 3 weeks, until week 36 once every two weeks and from week 36 onwards you will visit the midwife once a week. There are a number of routine matters that occur at every check-up, namely measuring your blood pressure, weighing your body weight, feeling your abdomen and listening to the heartbeat. Read more about planning during your pregnancy here.

Week 17

180 mm / 18 cm and 200 grams
From now on, your baby will gain more body fat, which will keep it warm in the future. Babies mainly have brown fat, this fat does not store calories. Adults mainly have white fat, which does retain calories. Your baby can also hear outside noise. The eyes are still closed. The baby is now growing about 20 mm/2 cm per week. The blood circulation in your own body is now much better and this also applies to the blood circulation in your genitals. This can make you more likely to become aroused and desire sex. Your round breasts and stomach may also give you a sexy feeling. You don’t have to worry about damaging the baby, the uterus can contract after sex, which also doesn’t do any harm. There may be slight blood loss, as the uterus has better blood flow and the superficial blood vessels can burst in contact with the penis, this is also not a problem.

Week 18

200 mm / 20 cm and 200 grams
Your baby weighs approximately two ounces. Your uterus still offers enough space to move freely. The baby’s muscles are so strong now that you might feel the kicking this week. Although it may still take a while. The baby’s sense of touch has also now been developed. This means that from now on it can sense when you stroke your stomach and it can also sense its own movements. The muscles of the eyes are also trained in the meantime, the baby cannot look straight ahead, but it does get some practice. Very occasionally it looks straight, but in most cases not. Even after birth, your child will still have difficulty fixing their gaze. From now on the brain also starts to grow very fast. It is good to know that your baby has its own blood circulation, separate from yours. There is an old-fashioned expression that every pregnancy costs a tooth, because the baby would remove calcium from your teeth. Fortunately, this is just a fable. However, you may experience blood more quickly while brushing, this is due to the influence of hormones. The growth of bacteria in your mouth can also be inhibited less effectively (this easily causes plaque to form). So brush with a soft brush and take good care of your teeth. You can visit the dentist one more time, but it doesn’t matter that you are pregnant. He can then place a special apron over your stomach if an X-ray needs to be taken.

Week 19

220 mm / 22 cm and 250 grams
Tiny stubs form in the jaws where teeth and molars will later be located. From now on, your child grows at its own pace, before then weight and height are approximately the same for all babies. The downy hairs it got a few weeks ago are now slowly starting to fall out again. In their place are some firmer hairs. Especially on top of the head. Most women get more energy during this period, are you still tired? Then it may indicate iron deficiency. You often feel dizzy and pale. Ask your doctor or midwife if your Hb level can be measured. Iron deficiency is a common ailment in pregnant women, and nothing to worry about.

Week 20

250 mm / 25 cm and 320 grams
You are half way there! His skin becomes increasingly thicker at twenty weeks. There is still that waxy layer of sebum present on top of this, also known as vernix. Vernix protects its skin while it happily swims around in the amniotic fluid. The baby is full of life and moves about 200 times a day. There is a good chance that you will now feel the movements. One person describes it as a rumbling in the intestines, another as a butterfly in the stomach and the third as a bursting soap bubble. Have you felt anything at all this week? Don’t worry, sometimes it just takes a little longer. You can also forget the myth that you have to eat for two during pregnancy. If you just continue to eat healthy and responsibly, you don’t have to worry that the baby will lack anything. It is recommended: 5-7 slices of brown bread, 3-5 potatoes or tablespoons of rice/pasta, 3-4 tablespoons of vegetables, 2 pieces of fruit, 3 glasses of milk or milk products, 100 grams of meat/chicken or meat substitute, 1-2 slices of meat products, 35-45 grams of butter and 2 slices of cheese. Are you unsure whether you can eat certain foods or not? Then read more here.

read more

  • Development of your baby weeks 21 to 42
  • Pregnant, what can I do and what can I no longer do?
  • What to eat/drink/do during your pregnancy
  • Facts and Fiction about pregnancy
  • Tips to combat pregnancy ailments