For most women, especially first-time moms, it's almost impossible for anyone to tell they're pregnant during the first trimester. As a brand-new expectant mother, you're not showing much, if at all, and the only telltale outward sign might be that smile you just can't suppress.
But inside, both your baby and your body are already working at top speed, like the Apple factory before a new iPad launches. During the next 13 weeks, your baby will:
Grow from a tiny cluster of cells called a blastocyst (about the size of the head of a carpenter's nail) at week three of pregnancy to about 3 inches long (think the length of your car key) by week 12.
Develop pigment in her eyes (still hidden behind sealed lids), form a tiny tongue with taste buds, and build a full four-chambered heart beating at about 180 beats per minute.
Form all of her major organs and body systems -- a critical time of structural development. The period between eight and 10 weeks' gestation is perhaps the single most crucial time for fetal development, says Annette Perez-Delboy, MD, assistant professor of obstetrics and gynecology at Columbia University Medical Center and a maternal-fetal medicine specialist.
Meanwhile, there's a lot going on with you, too:
Your heart is rerouting much of its effort toward baby's temporary digs, your uterus. By the end of the first trimester, a significant amount of your cardiac output goes to the uterus.
Your uterus is expanding from the size of a closed fist at conception to about the size of a small melon at 13 to 14 weeks.
You may be noticing some of the first physical signs of pregnancy: breasts that are sore or tingle at the slightest touch, skin that's drier or oilier than usual, and "morning sickness" -- which may or may not restrict itself to the morning hours. As many as 70% to 80% of pregnant women have it, but not feeling morning sickness doesn't mean there's anything wrong with the baby.