Using ultrasounds, doctors can measure fetal development throughout the pregnancy process to ensure a healthy birth. A pregnancy officially starts at the beginning of the last menstrual period of the mother, rather than the moment of conception, and usually lasts about 40 weeks until the point of birth. The pregnancy is divided into trimesters, each lasting roughly three months (around 13 weeks), with the third trimester generally lasting an additional week. With an ultrasound, expecting parents can get a glimpse of their baby through various changes and development.

Week 6:

About halfway through the first trimester, the embryo is nearly a quarter inch in length, blood vessels are beginning to form, the heart begins to beat, and the eyes are beginning to develop. Arm and leg cells have been established, and the brain starts to expand. Though the fetus still appears quite small, many major organs have already started their development process.

Week 9:

The fetus now has defined digits on both the hands and feet, though the webbing around them has not yet receded, and cartilage has begun to form a structure for the child. The heart has essentially been completed, and the tongue and voice box have started developing. During the first nine weeks, the brain develops rapidly, and at this point brain activity can be measured.

Week 13:

Now the fetus has sextupled in size since the sixth week, measuring at about three inches in length. Bones have started to develop in place of cartilage, and facial features begin to appear. The fetus can now move around and ultrasound pictures may reveal defined external genitalia, showing the gender of the child. The child may move around and kick inside the womb.

Week 18:

The baby has now doubled in size. Its length is about six inches, and it weighs around seven ounces. Movement becomes more recognizable, and the child has a greater ability to interact with the environment outside the womb. He or she will be able to hear sounds, and will have specific rest and activity intervals. The baby may begin to cry, but without air cannot create noise.

Week 22:

The baby’s eyebrows and eyelids have now fully formed, and the baby may now react to particularly loud noises outside of the womb. The fetus has now become recognizable as a baby, but still has some growing to do.

Week 26:

The baby is over a foot long, and almost two pounds. Its hands have become active, and the child now has a definite sense of vision. It has begun to move as if breathing, but still receives all its nutrients through the umbilical cord. Mothers may be able to interact with their child through their belly wall.

Week 32:

The baby is now almost a foot and a half in length and around four pounds, and has started to develop a layer of fat beneath its skin for protection. Its finger and toenails have formed, and its head has increased to a diameter of roughly four inches. Because the baby is now so large, it has less room to move around, and will likely sleep a majority of the time.

Week 36:

At this point, the baby should be growing about half a pound a week as it develops thicker layers of fat, which makes it look chubby around its arms and neck. All organs, except the lungs, have usually reached maturity, and the kidneys and liver have started filtering waste. This is the final week before the normal birthing window.

Week 40:

The baby has likely completed fetal development and is ready for delivery. It now sits in its final birth position and, though there is very little room, will likely continue to kick until the point of birth.