WEDNESDAY, March 13, 2002 (HealthDayNews) -- Fraternal twins, who share a womb but not a placenta, are as likely to survive the early stages of pregnancy as a single embryo.
Australian researchers report that among women who conceive fraternal twins, there is no evidence of an increased risk of losing one of the twins during early pregnancy. This challenges the longstanding concept of competition between fraternal twins. The findings appear in tomorrow's issue of the journal Nature.
Unlike identical twins, which share a placenta, fraternal twins come from two fertilized eggs and are nourished by individual placentas. Each egg is released by a single follicle, which then becomes a hormone-producing, pregnancy-supporting organ called the corpus luteum.
The corpus luteum is an endocrine gland that forms every month following the release of an egg from the ovary. The gland produces estrogen and progesterone, preparing the lining of the uterus to receive a fertilized egg should conception take place. If an egg is fertilized, the corpus luteum remains for most of the first trimester, but if conception doesn't take place, hormone production by the gland drops off and menstruation occurs.
Dr. Anna K. Parsons, an associate professor of reproductive endocrinology at the University of South Florida College of Medicine and an expert on corpora lutea, says the likelihood of a woman double-ovulating increases as she approaches the age of 35, then drops off. However, she adds, "often one corpus luteum is a poor one." Moreover, she says, that by age 35 a woman's eggs are less able to develop into a healthy fetus.
Parsons says it's very easy to recognize the corpus luteum using ultrasound if the practitioner looks for the gland's heightened blood supply.
In the latest study, doctors at Monash University in Victoria, Australia, hoped to determine how often they could identify ovulation and the corpus luteum using ultrasound.
According to senior investigator Beverley Vollenhoven, the researchers decided to look at how many women who double ovulate, developing two corpora lutea, end up with a twin pregnancy.
From 504 early pregnancy ultrasound scans of women during early pregnancy, the researchers identified 48 cases of double ovulation. The average maternal age among the women who double ovulated was 32.5 years.
Out of the 48 cases, 27 occurred spontaneously, resulting in nine sets of twins and 18 single births. Another 15 women double ovulated following treatment with clomiphene citrate, a fertility drug that boosts the likelihood of double ovulation from 2 percent to 4 percent. Among those women, three gave birth to twins, placing the overall probability of a second egg being fertilized and developing at between 20 percent and 30 percent.
That range of probability matches that of a single egg being fertilized and growing to term.
Vollenhoven admits she and her colleagues were surprised by their discovery.
"If you have a single baby or a twin pregnancy, there is no difference in the chance of one or two of them developing," she says.
The findings cast doubt on the widely held belief that the second fertilized egg is more likely to die during the early stages of pregnancy.
At the same time, the study doesn't rule out the possibility that there is a greater risk of losing both twins compared to losing a single pregnancy.
"There's always been [an idea] that, particularly later on in pregnancy, that one twin may be competing with another for nutrition," Vollenhoven says. "There also has been some thought that this may be something that occurs early on as well."
When using in vitro fertilization, doctors don't like "putting back more than two embryos, for example, because we know that multiple pregnancies are an issue later in pregnancy," Vollenhoven says. "But now we've shown that there's no difference in developing between one embryo and two embryos."
Parsons says the study will highlight the importance of the corpus luteum in ovulation and pregnancy. "I'm glad that this article is out," she says.
"Dizygotic [fraternal] twins are an abundance of nature," Parsons says. "The competition between the two types of twins must be very different, because [identical] twins share a placenta, and the dizygotic twins are independent, and really, they're mostly competing for space."
The Australian researchers are now looking at the size of the corpus luteum in normal and abnormal pregnancies and studying the genetic information of the twins in the study.
"Ultrasound has been shown to be fairly accurate now in looking for the corpus luteum and finding twins very early in pregnancy," says Vollenhoven. "What we can also hopefully show is that if our [genetic] testing is correct, that in the future, ultrasound may be sufficient to tell us that these are going to be non-identical twins."
What To Do
Learn about twins from TwinStuff.com or the Virtual Children's Hospital.
You can also check out this information on ultrasound during pregnancy from Dr. Michael Applebaum.
