Small for gestational age is a term used to describe babies who are smaller than expected for the number of weeks of pregnancy. These babies have a birth weight below the 10th percentile.
SGA babies may appear physically and neurologically mature but are smaller than other babies of the same gestational age. SGA babies may be proportionately small (equally small all over) or they may be of normal length and size but have lower weight and body mass. SGA babies may be premature (born before 37 weeks of pregnancy), full term (37 to 41 weeks), or post term (after 42 weeks of pregnancy).
When the unborn baby has trouble getting enough oxygen or nutrients during pregnancy, the baby’s body and organs don't grow as much as they should. Some of the problems that cause babies to be small for gestational age limit how much blood flows through the placenta. This can cause the baby to get less oxygen than usual (or expected) This increases the baby’s risks during pregnancy and delivery, and later.
What causes babies to be small for gestational age?
Although some babies are small because of genetics (their parents are small), most SGA babies are small because of fetal growth problems that occur during pregnancy. Many babies with SGA have a condition called intrauterine growth restriction (IUGR). IUGR occurs when the fetus does not receive the necessary nutrients and oxygen needed for proper growth and development of organs and tissues. IUGR can begin at any time in pregnancy. Early-onset IUGR is often due to chromosomal abnormalities, maternal disease, or severe problems with the placenta. Late-onset growth restriction (after 32 weeks) is usually related to other problems.
Babies with SGA and/or IUGR may have problems at birth including the following
- Decreased oxygen levels
- Low Apgar scores (an assessment that helps identify babies with difficulty adapting after delivery)
- Meconium aspiration (inhalation of the first stools passed in utero) which can lead to difficulty breathing
- Hypoglycemia (low blood sugar)
- Difficulty maintaining normal body temperature
- Polycythemia (too many red blood cells)
How are small for gestational age babies diagnosed?
Babies with this problem are often diagnosed with FGR before birth. During pregnancy, a baby’s size can be guessed in different ways. The height of the top of a mother’s uterus can be measured from the pubic bone. This measurement in centimeters often links with the number of weeks of pregnancy after the 20th week. If the measurement is low for the number of weeks, then the baby may be smaller than expected. Other tests used for diagnosis may include:
- Ultrasound to estimate the baby’s size
- Doppler flow to help check blood flow to the baby during pregnancy
- Mother’s weight gain to tell how a baby is growing during pregnancy
- Baby’s birth weight as compared with the gestational age once the baby is born. The healthcare provider may use a formula to figure out the baby’s body mass.
Small for gestational age prevention
- Antiplatelet agents may be effective in preventing SGA birth in women at high risk of pre-eclampsia, although the effect size is small.
- In women at high risk of pre-eclampsia, antiplatelet agents should be commenced at, or before, 16 weeks of pregnancy.
- There is no consistent evidence that dietary modification, progesterone or calcium prevents birth of an SGA infant. These interventions should not be used for this indication.
- Interventions to promote smoking cessation may prevent delivery of an SGA infant. The health benefits of smoking cessation indicate that these interventions should be offered to all women who are pregnant and smoke.
- Antithrombotic therapy appears to be a promising therapy for preventing delivery of an SGA infant in high-risk women. However, there is insufficient evidence, especially concerning serious adverse effects, to recommend its use.
