The first trimester, or three months, of pregnancy, can present many risks to the developing fetus. Because organs, facial features, skeletal tissue and limbs are developing at this time, any negative affect on the fetus can result in serious birth defects. While most risks to the pregnancy can't be changed, because they're caused by genetic abnormalities or implantation problems, some risks to early pregnancy can be modified by lifestyle changes.
Miscarriage is common in the first trimester; up to 20 percent of all diagnosed pregnancies end in miscarriage, most before 13 weeks. At least 50 percent of all miscarriages are caused by chromosomal abnormalities and are not preventable, according to the American Congress of Obstetricians and Gynecologists (ACOG). Many miscarriages are the result of blighted ovum, an abnormal pregnancy in which the placenta and gestational sac grow but no embryo develops. Smoking, using illegal drugs and drinking alcohol in the first trimester all increase the risk of miscarriage.
Ectopic pregnancy—pregnancy implanted outside the uterus—occurs in one in 40 to 100 pregnancies. The most common site of ectopic pregnancy is the fallopian tube, but ectopic pregnancies can implant in the abdomen, ovary or cervix. Pelvic inflammatory disease, endometriosis and tubal ligation reversal all increase the risk of ectopic pregnancy. Ectopic pregnancies are not viable because the space where they implant isn't large enough to sustain their continued growth, in most cases, although occasionally abdominal pregnancies survive. An ectopic pregnancy is usually diagnosed when no fetal tissue is seen in the uterus on ultrasound or because pain develops.
Ectopic pregnancies must be dissolved surgically or by taking chemotherapy drugs such as methotrexate. The tube itself may also need to be surgically removed. A ruptured ectopic pregnancy is a medical emergency and needs immediate treatment to save the mother's life; uncontrollable bleeding may occur quickly.
The growing fetus is most at risk for acquired birth defects between the eighth and 13th weeks of pregnancy. Exposure to harmful substances such as drugs, alcohol or toxins in this vulnerable time can cause birth defects, as can illnesses such as cytomegalovirus, rubella, syphilis, toxoplasmosis and occasionally chicken pox, according to the Penn State Children's Hospital. But birth defects can also occur spontaneously. Birth defects occur in one out of 33 pregnancies, and most occur in the first three months of pregnancy, the Centers for Disease Control states.
Common birth defects that develop in the first three months include heart defects, which comprise between one-third to one-fourth of all birth defects, neural tube defects, including spina bifida and anencephaly, or lack of brain tissue. These affect one in 1,000 births. Facial defects such as cleft lip and palate affect one in 700 to 1,000 births. Taking 400 mg of folic acid daily and avoiding alcohol, quitting smoking and not taking illegal drugs help reduce your chance of birth defects.