The study of very early pregnancy loss is impractical in the general population, but possible amongst infertility patients receiving carefully monitored treatments. We examined the association between fetal loss and the risk of birth defects in the surviving co-twin in a retrospective cohort study of infertility patients within an infertility clinic in South Australia from January 1986 to December 2002, linked to population registries for births, terminations and birth defects. The study population consisted of a total of 5683 births. Births from singleton pregnancies without loss were compared with survivors from (1) pregnancies with an empty fetal sac at 6–8 weeks after embryo transfer, (2) fetal loss subsequent to 8-week ultrasound and (3) multiple pregnancy continuing to birth. Odds ratios (OR) for birth defects were calculated with adjustment for confounders. Amongst infertility patients, the prevalence of birth defects was 7.9% for all twin pregnancies without fetal loss compared with 14.6% in pregnancies in which there had been an empty sac at ultrasound, and 11.6% for pregnancies with fetal loss after 6–8 weeks. Compared with singleton pregnancies without loss, the presence of an empty sac was associated with an increased risk of any defect (OR=1.90, 95% confidence intervals (CI)=1.09–3.30) and with multiple defects (OR=2.87, 95% CI=1.31–6.28). Twin pregnancies continuing to birth without loss were not associated with an overall increased prevalence of defects. We conclude that the observed loss of a co-twin by 6–8 weeks of pregnancy is related to the risk of major birth defects in the survivor.

Ultrasonographic examinations were performed on 519 pregnant women in the first trimester at three hospitals in Tokyo from August 1985 to March 1987. Among 519 women, two fetal heart movements (FHMs) were comfirmed in six cases. In two of these six cases, one of two FHMs disappeared two weeks later, and these were considered the “vanishing twin”. In eight cases among those with ultrasonographic image of echo-free-space (EFS) in the uterus, the observed EFS was considered to be a probable empty gestational sac (GS-like-EFS-image). Seven of eight GS-like-EFS-images were observed in October-December (p < 0.01) and two “vanishing twin” cases were also observed in the same season. Spontaneous abortion occurred in 11 cases and these were also more frequently observed in October-November (p < 0.05). Some abortive factor is supposed to have prevailed in October-December. All six women having twin fetuses (including the “vanishing twin” cases) were born in January-May (p < 0.05) and those having GS-like-EFS-images or terminating their pregnancy in spontaneous abortion were also born more frequently in the same season. Twin-prone and abortion-prone characteristics of the women born in this season are considered.

Serial ultrasound examinations have demonstrated that one of two gestational sacs in a twin pregnancy may often disappear. When it disappears at an early stage of gestation, the pregnancy may advance without any disturbance and the cotwin can be delivered well developed and lively. When the intrauterine death occurs in the second trimester, the dead fetus usually results in a fetus papyraceus and the cotwin continues to be alive near term. However, when death occurs in the last trimester, the viable twin may be spontaneously delivered soon and be premature. In some cases of late fetal death, the dead fetus may induce intravascular thromboses in many organs of the surviving cotwin, so that the living infant may develop cerebral palsy later after birth.

The relation between spontaneous abortions and twinning was studied in the years 1980-1981 in 22 towns of the South Moravian Region, Czechoslovakia. Each town has more than 10,000 inhabitants and less than 750 births per year. The number of spontaneous abortions and the number of twin births were evaluated separately for each town and for a time unit of three months. The comparison indicated that the group with a higher number of spontaneous abortions also had a lower number of twin births. The relation between spontaneous abortions and twinning is discussed.