Pregnancy by the Week


Expectant parents love flipping pages in their pregnancy books or reading online about their pregnancy by the week. There is plenty of information available on each week of development, including the growth rate and size of the embryo or fetus, physical development, and the physical effects of pregnancy on the mother.

Each week of pregnancy brings new changes and developments, helping parents learn more about their growing baby and prepare for the birth.

Pregnancy by the Week


Ovulation and Conception

The weeks of pregnancy are actually counted starting at the date of the last monthly period, so by the time a pregnancy test comes out positive the pregnancy is about 4 weeks along. Ovulation occurs around 2 weeks, soon followed by fertilization by the sperm and the conception and implantation of the blastocyst that will develop into the embryo.

The pregnancy hormone hCG is produced, which is what will make a pregnancy test come out positive by week 4. Early pregnancy symptoms like fatigue, breast tenderness and swelling, nausea, and frequent urination may already be present in some women.


First Trimester

At about a month, the undeveloped phase of pregnancy, enduring through 10 weeks, starts. The embryo starts off the size of a poppy seed at 4 weeks but will grow to be 3 inches long and weigh 1 ounce by the end of the first trimester, at 13 weeks.

Development during the weeks of pregnancy in the first trimester is crucial to the survival of the embryo and viability of the pregnancy. The baby’s heart and circulatory system begin to form, and the nose, mouth, and ears take shape; his bones, intestines, lungs, and major glands and organs also begin to take form.

Hands and feet start to emerge during week 7, followed by fingers, toes, full eyelids, and neural pathways in week 8.

By week 9 of the pregnancy, all of the baby’s major parts are formed–her heart has divided into its 4 chambers, most of the organs, muscles, and nerves are beginning to function, and the external sex organs are formed.

At week 10 of pregnancy, the fetal period begins, during which the baby will grow rapidly, along with all of his tissues and organs. Reflexes like the curling of toes, opening and closing of fingers , and a sucking motion of the mouth all begin in week 12 of pregnancy.

In the last week of the first trimester the baby develops fingerprints, and his body is beginning to overtake the head in growth rate.


Second Trimester

At the start of the second trimester in pregnancy, the baby is the size of a lemon, and her facial muscles can form all types of facial expressions, and may even be able to suck her thumb. The kidneys are making urine, and the small hands can grasp.

By week 15, the baby can move all of his limbs and joints, and the sex of the baby may be determined by ultrasound. During week 17, the skeleton of the baby will begin to turn from cartilage to bone, and the placenta will start to growstronger.

By week 18, the baby’s near-constant kicks and flexes may be felt by the mother, and at 19 weeks the baby may be able to hear his parents’ voices. In week 22, the eyes are forming, but the irises still lack color and, beginning in week 23, it may be possible to see the baby as it moves in the mother’s womb.

The second trimester of pregnancy by the week ends in week 27, with the baby weighing in at nearly 2 pounds and measuring 14 1/2 inches in length. Her brain is active and her lungs are functional, her eyes open and close, and she is putting on some much-needed baby fat.


Third Trimester

By the start of the third trimester of pregnancy, the baby can blink his eyes, and his vision will continue to develop over the coming weeks. Beginning at 34 weeks, if your baby is born preterm, he has an excellent chance of surviving and thriving with no lasting health problems.

At 35 weeks, all of the baby’s development is nearly complete, so the next several weeks will be spent mostly gaining weight. A baby is considered full term at 37 weeks, his lungs are mature, and he may weigh around 6 1/3 pounds, and measure 19 inches in length.

While 40 weeks is considered the average length of pregnancy, most babies are born between 37 and 42 weeks.



While babies do will in general create along these rules, they are simply gauges and midpoints. Not all babies will be the suggested weight or length at any given stage during the pregnancy, and that is entirely normal.

All babies have their own individual characteristics, even in the womb.


Fetal Presentation

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