Fetal development begins before you even know you're pregnant, although technically the beginnings of your unborn child are called a zygote. A zygote is a fertilized ovum and occurs within 24 hours after the ovaries release an egg into one of your fallopian tubes. Usually only a single egg is released but sometimes two eggs are released and fertilized at the same time. When this happens, fraternal twins are born.
Baby development moves towards the embryonic stage around week five after the fertilized egg has implanted itself in the uterine wall. This stage lasts until the end of week eight and fetus development images will show stunning changes in embryo development during these 21 days. At the beginning of the embryonic stage your baby will be the size of the tip of a pen but will have three very specific layers where your baby's central and peripheral nervous systems, bones and muscles, and lungs and intestines will form. Your baby won't look like much at this point.
By week six your baby's heart will be pumping blood and the most rudimentary facial features will appear. Your baby will start to look like something besides a worm although she won't be recognizable as a human yet. Her body will take on a c-shaped curvature. Around week seven tiny nostrils are visible and eye lenses start to form. At the end of the week eight your baby's body will start to straighten and she'll start to move around a lot even though she'll be much too small for you to feel it. She'll be about ½ inch long. At week nine your baby will be considered a fetus. By the 12th week she'll have a human profile.
Women who do not wish to be pregnant may choose an abortion before the first trimester is finished. The procedure can be done into the second and rarely the third trimester but the majority of pregnancy terminations are done before the 12th week of pregnancy.
Fetus Development in the Second Trimester
According to the pregnancy calendar, the second trimester starts at 13 weeks. A mere two weeks later and your baby might begin sucking his own thumb and is developing reflexes. The sex is identifiable and fingers and toes are well defined. Tooth buds are forming and so are sweat glands on the palms and soles. The baby looks distinctly human but is still not able to survive outside the mother's body.
By the end of week 20 your baby will weigh about a pound. Eyebrows, eyelashes and eyelids appear and the internal organs are maturing. You might be able to feel fetal movement by now. Your health care practitioner may also order the first ultrasound to check for anomalies in your developing baby. In many pregnancies this is the only diagnostic ultrasound required unless anomalies are detected, you're giving birth to multiples or there's potential complications with the pregnancy that need to be closely monitored like problems with the placenta.
Your baby will be able to hiccup by the end of the second trimester. He'll be able to open his eyes for short periods of time and he'll weigh about two pounds.
Fetal Development in the Third Trimester
If your baby were to be born at 28 weeks, around the beginning of the final trimester, his survival rate will be about 90 percent with intensive medical care. The following week your baby's bones are fully developed and she'll begin storing calcium, iron and phosphorous. At 33 weeks your baby will be able to detect light and will have been practicing breathing for a week or two now. Rapid weight gain occurs from week 36 and at 37 weeks your baby is considered full term. He could be born anytime between now and 42 weeks gestation so it's a good idea to be prepared for his arrival any day and make sure you know the signs of labor.