Fetus Stages and Development Features
Pregnancy officially begins the first day of the last period you had before you conceived, even though there was no baby or potential baby in the works. Conception occurs two weeks after your menstruation and the fertilized egg that travels from the fallopian tubes to the uterus is called a zygote. When this zygote goes through more cell changes and divisions and attaches itself to your uterus wall, your developing baby reaches the embryonic stage. Your baby is medically considered an embryo until either eight weeks or 12 weeks gestation depending on your source.
There's some disagreement between the medical community and some pregnancy journals and publications about the actual start of the fetus stages. The official fetus definition according to a medical dictionary (MedineNet.com) is "the unborn offspring from the end of the 8th week after conception (when the major structures have formed) until birth. Up until the eighth week, the developing offspring is called an embryo."
For the purpose of this article, we'll go with the medical dictionary definition of a fetus and look at some of the changes in a developing fetus from nine weeks gestation.
Nine Weeks Gestation
At nine weeks your baby is approximately the size of a large grape or one-inch long. Your baby has started moving around and has lost the embryo tail. He or she will have sex organs but they won't be visible yet with an ultrasound. The heart will have been beating for close to three weeks now and will have divided into four separate chambers.
Fetus pictures at this stage will show something that loosely resembles a human in appearance but with an incredibly large head and nubs for arms and legs. If you were to get an ultrasound at this point, you might need a technician to point out the parts to you. But once they're pointed out you'll be able to clearly see your developing baby and his or her beating heart.
Women who do not want to be pregnant may choose to have an abortion performed at around this time in pregnancy. Generally women who choose abortion get the procedure done between the sixth and 10th week of pregnancy depending on when they find out they're pregnant and how soon someone is available to perform the abortion.
By the end of the 12th week of the fetus stages of growth, and the end of the first trimester, your baby's development has progressed far enough that your healthcare practitioner might be able to hear your baby's heartbeat with a fetal Doppler. Arms and legs are prominent enough to see clearly in an ultrasound.
Baby Development and Viability
Around 24 weeks your developing fetus has grown enough to survive outside of your uterus if she were to be born prematurely. But the odds would not be high and she would need extensive medical care and would be in the hospital's Intensive Care Unit probably until her original due date.
By week 27 your baby's rate of survival has jumped to 80 percent if she were to be born prematurely now. Serious complications are still possible because your baby isn't fully developed. She'll be approximately two pounds now and will be able to blink her eyes.
The Developing Fetus in the Third Trimester
Around week 30 and a couple of weeks into the third trimester your baby will have the ability to control his own temperature but still needs to put on several layers of fat in order to greatly reduce the chance of medical intervention when born. At week 33 your baby will be drinking a pint of amniotic fluid daily and might have fingernails long enough to scratch himself before birth. By week 35 90 percent of premature babies survive with minimal medical intervention.
As your baby gets larger, your uterus will expand to accommodate him. The bottom part of the uterus is said to do most of its stretching in the third trimester. As it stretches, the placenta is carried upwards with the change in uterus size since it's attached to the uterine wall. In a small percentage of women, the placenta doesn't move up especially if it's been discovered in an earlier ultrasound that the placenta is covering the cervix. If the placenta is low lying, a c-section may be necessary when it's time to give birth.
A full term birth can happen anytime from 37 weeks and on. While a developing fetus may be considered a full-term baby at this time, most are born closer to the 40-week mark or even later.