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Prenatal Development

What is prenatal development?

Pregnancy is an exciting time. As you prepare for the arrival of your baby, you may decorate the nursery, think of baby names, and start preparing your finances for a new addition. Your body will change over the next nine months, and as your due date nears, you’ll want to keep close track of your baby’s growth and development.

Prenatal development starts at conception and ends with the birth of your baby. It takes about 40 weeks or nine months to create a new life, and your pregnancy is broken into three, 12-week trimesters. Each trimester brings about new changes and developments.

The first trimester

Most women ovulate once a month, about two weeks after their last menstrual cycle. Ovulation is the release of an egg. If you have sexual relations before, during, or within 24 hours of ovulation, sperm can travel from the vagina into your fallopian tubes and fertilize the egg. Sperm can live in the body for up to five days, so your chances of conceiving are higher if you have intercourse in the days leading up to ovulation.

The first trimester is the first 12 weeks of your pregnancy. This is also the embryonic stage, which continues through the 10th week of your first trimester. Your baby is called an embryo at this stage. Since you're early in the pregnancy, you may experience early pregnancy symptoms, such as:

  • morning sickness
  • increased urination
  • swollen breasts
  • fatigue

It’s during the embryonic stage that embryo cells begin to multiply and form your baby’s body.

The embryo’s gastrointestinal tract, spinal cord, heart, and brain are the first organs to develop. This starts about one week after conception.

The placenta also develops in the embryonic stage and it plays a vital role in pregnancy because it carries nutrients, oxygen, and water to your baby. This development takes place about two or three weeks after conception.

Your baby’s body also begins to form. This includes development of:

  • facial features
  • eyes
  • ears
  • the nasal passage
  • arms
  • legs
  • hair follicles
  • mouth
  • taste buds

These developments continue up to week 10, which is the end of the embryonic stage. Your baby’s sex organs will have formed by the end of this stage, although it's too early to determine the baby’s sex on an ultrasound.

The second trimester

Your second trimester of pregnancy begins at week 13. At the end of the embryonic stage, your baby is about 3 inches long and weighs about 1 ounce. Once the embryonic stage ends, your baby enters the fetal stage.

Around weeks 12 through 14, the fetus begins sucking and swallowing, as well as moving inside the womb, although it may be too early to feel these movements. The fetus will grow to about 6 inches during these two weeks.

As your baby’s muscles develop and grow, you'll begin to notice movement around weeks 15 through 18. It’s also around this time that your baby’s skin develops a white substance called vernix, which protects the skin from amniotic fluid. Your baby will begin making facial movements inside the womb, and you may catch a glimpse of the fetus appearing to smile or squint on a 3-D ultrasound.

The fetus’s middle ear typically develops at week 20, and with this development your baby can hear your heartbeat and voice.

During the final weeks of your second trimester, fat starts to develop on your baby’s skin, and its major organs — such as the liver and pancreas — become stronger. At the end of 24 weeks, the fetus is about 10 to 11 inches long and weighs about 1 pound.

The third trimester

You’ve finally reached the last 12 weeks of your pregnancy and you're nearing the finish line! By your sixth month your body experiences a lot of changes, such as weight gain, heartburn, and you may have trouble sleeping. But while you may be uncomfortable at times, you're elated and amazed by your baby’s progress and development.

Your final trimester begins 25 weeks after conception. All of your baby’s organs have developed, and movement inside the womb increases over the next couple of months. Sometimes, it may feel as if you're being punched and kicked in the stomach. But that's just your baby exploring its surroundings.

The baby’s lungs are not fully developed in the early stages of the third trimester, but it may weigh about 4 pounds and be able to recognize changes in sound. The baby is surrounded by darkness, but it can detect bright lights from outside the womb. The lungs fully develop after 28 weeks or seven months.

Between weeks 31 and 34 weeks after conception, your baby begins preparing for birth and gradually moves into a head-down position. Since you're nearing the home stretch, the fetus will grow rapidly over the final weeks and you may notice less movement. Don't panic if your baby doesn’t move as often. Since the fetus is getting bigger every day, it doesn’t have as much room in the womb to move around. But if you have concerns, don't hesitate to contact your doctor.

The final weeks of the third trimester — and your pregnancy — begin 35 to 38 weeks after conception, which is 37 to 40 weeks since your last period. You’re considered full-term 36 weeks after conception (or 38 weeks after your last period). Your baby’s organs are developed, the fetus is fully matured, and you can deliver at any day. At this stage of prenatal development, your fetus is about 19 to 20 inches long and weighs between 6 and 10 pounds.

You’re close to delivering the baby when labor contractions intensify and occur every five minutes.

Labor and delivery

Your doctor estimates your due date in the pregnancy based on the date of your last menstrual cycle. Understand, however, that only 5 percent of babies arrive on their due dates.

Don't be alarmed if your baby doesn’t arrive by your due date. It’s common. You also shouldn’t panic if the baby arrives early. Your chances for a healthy pregnancy and delivery increase the longer the baby stays in the womb. However, you may find it comforting to know that a baby born as early as the beginning of the third trimester can survive without the help of medical technology.

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