Over the past 25 years, science has made great progress in helping women who are struggling to become pregnant actually conceive. Assisted reproductive technology or assisted conception, involves surgically removing the eggs from a woman’s ovaries, combining them with sperm in the laboratory, and then either returning the fertilized eggs to the original woman’s body or donating them to another woman. This process is commonly known as in vitro fertilization or IVF .

However, IVF is not without risk. For example, women who become pregnant through IVF have a much greater chance of multiple births (twins or more) than women who become pregnant naturally. But, does this tendency increase the risk of a poor health outcome for children born to women who used IVF? A study published in the January 23, 2004 issue of the British Medical Journal looked at natural and IVF conceptions to determine if there were differences in pregnancy outcomes.

About the study

The researchers conducted a systematic review of the medical literature and identified 25 studies comparing women who gave birth to either one child or twins. Of these, 17 studies used matched controls, meaning that every woman who became pregnant using IVF was compared to a woman with similar characteristics who became pregnant naturally.

The researchers compared the relative risk of a poor health outcome for babies born both naturally and through IVF. These poor health outcomes were defined as:

  • Low birth weight
  • Premature birth
  • Small size at birth
  • Need for a caesarean section
  • Admission to the neonatal intensive care unit of a hospital
  • Death

Relative risk is a measure of the association between an exposure and a health outcome. In this case, the exposure is IVF conception and the health outcome is poor health at birth. A relative risk of 1.0 would mean that a baby conceived through IVF had no greater or lesser risk of a poor outcome than a baby conceived naturally. A relative risk of 2.0 would indicate that a baby conceived with IVF had twice the risk of a poor outcome as compared to a baby conceived naturally. A relative risk of less than one would indicate that IVF conception had a protective effect. For instance, a relative risk of 0.80 would mean that IVF conception reduced the baby’s risk of a poor outcome by 20%.

The findings

The researchers found that IVF conception increased a single baby's risk for a poor health outcome, in some cases significantly. Single babies conceived through IVF were three times more likely to be very preterm and twice as likely to be preterm than single babies born to mothers who conceived naturally. They were also three times more likely to have a very low birth weight and almost twice as likely to have a low birthweight. The following table lists the relative risks for babies conceived through IVF in all health outcomes measured for both single births and twins.

Relative Risk for IVF Conception as Compared to Natural Conception

Poor Health Outcome

Single Births*


Very preterm (<32 weeks)



Preterm (<37 weeks)



Very low birth weight (<1500 grams or <3.3 pounds)



Low birth weight (<2500 grams or < 5.5 pounds)



Small for gestational age



Caesarean section



Neonatal intensive care unit



Perinatal mortality (stillbirth and death within first week)



*Results are from matched studies; non-matched studies showed similar results.

As illustrated in the table, the findings were not similar for twins conceived through IVF. In fact, one surprising result was that IVF conception appeared to have a protective effect in several outcomes for twins. For example, in twin pregnancies with IVF, the relative risk of perinatal mortality was about 40% lower than with natural conception.

How does this affect you?

The authors of the study concluded that doctors should inform all women undergoing IVF of the increased risk in single pregnancies. In addition, they strongly suggest that the focus of IVF be shifted from simply achieving pregnancy to achieving the birth of healthy children.

If you are having problems conceiving, should these findings keep you from trying IVF? Absolutely not; IVF has had many successes. But before making a decision, keep in mind that the ultimate reason for IVF is not to become pregnant, it’s to become a parent of a healthy baby. Setting your sights on this goal will allow you to make a more realistic decision among all the options at your disposal, including adoption.