The alarming decline in the global fertility rate has led to a dramatic increase in women seeking assistive reproductive treatments, giving a major boost to in vitro fertilization (IVF).

But selecting between frozen or fresh embryo transfer for IVF has remained a tough choice for millions of women trying for pregnancy. A new study comparing both strategies recommends the latter as the "gold standard" for women showing no exaggerated response to injectable hormones or ovarian hyper-stimulation syndrome.    

In the case of a fresh embryo transfer, patients are given hormones in order to stimulate egg production. Doctors retrieve the eggs, fertilize it and then place them in the mother to increase the chances of pregnancy.

But in the last few years, there has been a growing consensus on freezing all embryos known as "freeze all strategy" to boost the chances of pregnancy. The procedure involves transferring a frozen embryo collected from a previous IVF cycle into the uterus during a new menstrual cycle.

While the frozen embryos concept is fast catching up, studies comparing both assistive reproductive treatments have lacked evidence. In order to fill the gap, a team of researchers studied 460 women aged 18-39 years with regular menstrual cycles.

These women started their first, second, or third treatment cycle of IVF in which egg and sperm are placed in a Petri dish to fertilize or were getting intracytoplasmic sperm injection (ICSI) at fertility clinics at eight public hospitals in Denmark, Sweden and Spain.

"Time to pregnancy was longer in the freeze-all group," concluded the peer-reviewed study published in British Medical Journal (BMJ) on Wednesday.

Pregnancy rate—fetal heartbeat after eight weeks of gestation—was more or less the same in both procedures. While in the freeze-all group, the rate was 27.8 percent, in fresh transfer, it hovered at 29.6 percent.

No significant differences between the two groups were observed for pregnancy loss and none of the women had severe ovarian hyper-stimulation syndrome, said the study.

"A safe fresh embryo transfer strategy can be applied to women with regular menstrual cycles with strict cancellation criteria for the fresh transfer if an excess number of mature follicles are present," researchers said.

"The findings warrant caution in the indiscriminate application of a freeze-all strategy when no apparent risk of ovarian hyper-stimulation syndrome is present," they added.

However, previous research conducted in 2016 shows freeze all IVF boosting the birth rate among women suffering from a hormonal disorder.

A large Chinese randomized controlled trial involving 1,508 women having polycystic ovary syndrome (PCOS) showed a higher live birth rate after freeze-all compared with fresh transfer of 49.3 percent and 42 percent, respectively.

(Cover: Doctor Katarzyna Koziol injects sperm directly into an egg during in-vitro fertilization (IVF) procedure called Intracytoplasmic Sperm Injection (ICSI) at Novum clinic in Warsaw October 26, 2010. /Reuters)