A hidden twin, also called a vanishing twin or twin resorption, occurs when one twin dies in the uterus during pregnancy and is partially or completely reabsorbed by the mother and remaining twin. This results in a pregnancy that appears to be a singleton, but was originally a twin gestation. Hidden twins occur in up to 30% of all pregnancies, making them more common than one might think.
How Does a Hidden Twin Occur?
Most twins form very early in pregnancy when the embryo splits into two. In some cases, this split is incomplete and the two embryos fuse back together. In other cases, one embryo simply stops developing. This is known as vanishing twin syndrome. The remaining embryo continues to grow and develops into a healthy singleton baby. The mother may never know a twin pregnancy existed.
Sometimes a twin pregnancy is identified early on through ultrasound, but later scans reveal one fetus has been reabsorbed or “vanished.” This occurs in up to 24% of early twin pregnancies. It is usually caused by chromosomal abnormalities in one fetus, problems with the placenta or umbilical cord, or congenital defects that prevent further development.
How Common Are Hidden Twins?
Studies estimate hidden twins occur in between 21-30% of all twin pregnancies. This suggests hidden twins may be present in 2.4-5.2% of all pregnancies.
Some key statistics on the prevalence of hidden twins:
- Up to 24% of twin pregnancies identified by ultrasound early on later become singleton pregnancies due to vanishing twin syndrome.
- In one study of IVF pregnancies, 31.2% were twin gestations early on but only 22% were still twins by week 8.
- A review of studies estimated that among spontaneously conceived pregnancies, about 1 in 8 (13%) start off as twins but only 1 in 10 or 11 are twin births.
- Among pregnancies conceived through assisted reproductive technology like IVF, up to 36% may start as twin pregnancies and 21-26% end in twin births.
So while most pregnancies start off and remain singleton pregnancies, a hidden twin is surprisingly common.
When Does Vanishing Twin Syndrome Occur?
Vanishing twin syndrome most often occurs in the first trimester when the embryos are still developing and a heartbeat has not yet been detected.
- Weeks 4-7: Up to 75% of twin pregnancies result in vanishing twin syndrome during this time frame.
- Weeks 7-9: 25% of vanishing twins occur during these weeks right before embryonic development is complete.
- Weeks 10-12: Less than 3% occur during the end of the first trimester after the fetal period has begun.
The further along in pregnancy vanishing twin occurs, the more likely remaining fetal tissue will be found by ultrasound. Early on in weeks 4-7, the tissue is usually reabsorbed completely.
Certain factors can increase a woman’s risk of having a hidden twin:
Use of ART
Use of assisted reproductive technology like in vitro fertilization increases the chance of multiples. More twin pregnancies means more opportunities for one to vanish. Up to 30% of ART conceptions start as twins.
History of Miscarriage
Women who have suffered previous miscarriages may be at higher risk of vanishing twin syndrome. This is likely tied to underlying problems with implantation or placenta/cord development.
Older maternal age is associated with higher rates of embryonic chromosomal abnormalities, which can lead to twin demise. Women over 30 have a higher risk.
Ovulation induction with fertility medications often results in multiple follicle development, increasing the odds of conceiving twins and subsequent twin vanishing.
Signs and Symptoms
Hidden twins often cause no signs or symptoms, since the remaining fetus continues developing normally. Some subtle clues might include:
- Vaginal bleeding in the first trimester
- Cramping, pelvic pain
- A change from high to low hCG levels
- Disappearing fetus on early ultrasounds
- Fetal measurements that suddenly fall behind
However, keep in mind these symptoms can occur with any type of miscarriage and are not specific to vanished twins. The pregnancy may continue as normal with no complications.
Sometimes the twin demise leaves behind discernible tissue within the uterus. This is more likely to cause excessive bleeding, cramping, or changes in hCG levels.
In most cases, a hidden twin does not negatively impact the remaining twin or cause complications. However, there are a few risks to be aware of:
Vanishing Twin Syndrome
About 6% of vanishing twins are associated with defects in the surviving twin, such as limb abnormalities. The exact cause is unknown, but may be due to disruption in fetal blood supply.
Some studies show a slightly higher rate of preterm birth in singleton survivors of twin pregnancies. However, most research finds no increased risk if vanishing occurs early in pregnancy.
Low Birth Weight
Singleton babies born after a twin demise tend to have a normal birth weight unless vanishing occurs after week 20. Only very late twin demise results in smaller singleton babies.
The risk of cerebral palsy is approximately 1 in 1,250 for singletons. Vanishing twins appear to increase this risk to around 1 in 500 which is still quite low.
Diagnosing a Hidden Twin
Hidden twins are most often detected during early pregnancy ultrasounds when a twin pregnancy is seen but later scans show only one fetus. An ultrasound in the first trimester may reveal:
- Two gestational sacs, but only one viable fetus
- A fetus and an empty gestational sac
- A fetal sac with no embryo inside
- Signs of bleeding within the uterus
If you had ART, ultrasounds in the first few weeks can give important clues. Higher-than-expected hCG levels can also indicate a twin pregnancy.
Sometimes there are no early ultrasounds. Vanished twins might be discovered later in pregnancy if:
- Fetal measurements suddenly drop
- An abnormal placenta shape is seen
- Excess amniotic fluid is present
Unfortunately, hidden twins often go undiagnosed because early ultrasounds were not performed. The mother simply assumes she was always carrying a singleton.
Do Hidden Twins Affect Future Pregnancies?
Having a hidden twin generally does not affect the chances of twins in future pregnancies. Some exceptions:
- If ART was used, the chances are higher in subsequent pregnancies as well
- If an underlying health condition caused the twin demise, it may impact future pregnancies
- A family history of twins increases the odds going forward
- The statistics of having twins naturally rises with maternal age
Barring an underlying medical problem that caused the twin vanishing, future twinning risk is not typically elevated after a hidden twin.
When a hidden twin is discovered, especially later in pregnancy, additional ultrasounds and testing may be recommended:
- Growth assessments of the remaining fetus
- Checking the placenta and amniotic fluid volume
- Monitoring for preterm labor
- Nonstress tests later in pregnancy to ensure fetal wellbeing
Carrying a pregnancy with a vanished twin is usually considered higher-risk. However, with close monitoring, the outlook for a healthy singleton delivery is very good.
Learning you were originally carrying twins can bring up many emotions, even if vanishing occurred very early. Some women struggle with:
- Grief over the loss of one baby
- Guilt that they somehow caused the twin demise
- Fear that they will lose the remaining twin
- Sadness that they don’t get the twin pregnancy they hoped for
Processing these feelings with supportive loved ones or a counselor can help resolve the emotional impact. Support groups for twin loss can also provide comfort.
Hidden, vanished, or resorbed twin pregnancies occur far more frequently than most people assume. Up to 1 in 8 pregnancies start off as twins. While losing one twin can be emotionally difficult, the good news is that it rarely affects the health of the remaining baby.
With close monitoring, a hidden twin pregnancy usually continues as a healthy singleton pregnancy with good outcomes. Understanding how common hidden twins are provides reassurance to parents who experience it.