Stages of Pregnancy

1st Trimester Baby

The 1st Trimester Baby: 0‑14 Weeks

The 1st trimester (the first three months of pregnancy) is a critical time. It is the period of rapid growth and development of the fetus. It is also a time when the fetus is most at risk from hazards such as smoking, alcohol, drugs, infections, and X‑rays. By the end of the 1st trimester, all of the organs will be formed and functioning.

(Please note that although the term “baby” is used throughout this website, during the first 7 weeks the actual term for your developing baby is an “embryo,” and from week 8 until birth it is a “fetus.”)

Time	Weight and Length	Events
1 day		The sperm and ovum unite.
7 – 10 days		The fertilized ovum attaches to the lining of the uterus. The placenta begins to form.
2 weeks		The embryo is now a layered disc on the uterus wall. You will miss your menstrual period.
4 weeks	0.4 g (0.01 oz.)	4 weeks, 6 mm (1/4 in.) The beginnings of the embryo’s eyes, ears, nose, spine, digestive tract, and nervous system are present. The tube for the future heart starts beating.
8 weeks	22–24 mm (1 in.) 1g (0.036 oz.)	8 weeks, 22-24 mm (1 in.) The fetus now has all the organs that a full term baby will have. The heart is functioning. Bones begin to form.
12 weeks	9 cm (3 1/2 in.) 15 g (0.5 oz.)	12 weeks, 9 cm (3 1/2 in.) Tooth buds are present. Fingernails and toenails are forming. Immature kidneys secrete urine to the bladder. External genitalia are forming. The fetus can now move in the amniotic fluid, but you can’t feel it. Your health care provider may be able to hear the fetal heart rate with an electronic listening device.