Nigeria is placed third among the ten countries with the highest number of preterm births with 773,600, according to latest figures from the World Health Organisation (WHO).
Top on the list is India with 3,519,100; and China 1,172,300. Pakistan is fourth with 748,100; followed by Indonesia 675 700; United States of America 517,400; Bangladesh 424,100; Philippines 348,900; Democratic Republic of the Congo 341,400; and Brazil 279,300.
Preterm is defined as babies born alive before 37 weeks of pregnancy are completed. There are sub-categories of preterm birth, based on gestational age: extremely preterm (less than 28 weeks); very preterm (28 to 32 weeks); and moderate to late preterm (32 to 37 weeks).
According to the WHO, preterm birth occurs for a variety of reasons. Most preterm births happen spontaneously, but some are due to early induction of labour or caesarean birth, whether for medical or non-medical reasons.
Common causes of preterm birth include multiple pregnancies, infections and chronic conditions such as diabetes and high blood pressure; however, often no cause is identified. There could also be a genetic influence. Better understanding of the causes and mechanisms will advance the development of solutions to prevent preterm birth.
According to the WHO, more than 60 per cent of preterm births occur in Africa and South Asia, but preterm birth is truly a global problem. In the lower-income countries, on average, 12 per cent of babies are born too early compared with nine per cent in higher-income countries. Within countries, poorer families are at higher risk.
Meanwhile, In Vitro Ferlisation (IVF) may almost double the risk of women giving birth prematurely, according to a study of more than 60,000 pregnancies.
Ten per cent of women who conceived thanks to fertility treatment gave birth before 37 weeks, which is considered premature, scientists found. That is almost twice the rate for women who conceived naturally.
Tens of thousands of babies are born prematurely in Britain every year but survival rates are better than ever thanks to improving neonatal care.
However, those born before 37 weeks are still more likely than full-term babies to suffer from a range of long-term problems, including cerebral palsy, and developmental conditions such as Attention Deficit Hyperactive Disorder (ADHD). They are also less likely to excel at school.
The doctors behind the study think IVF may trigger changes in the placenta, which make premature birth more likely.
Experts believe that freezing the embryo first and then implanting it later, rather than doing everything in the same menstrual cycle, may offer protection against the risk.
More than 50,000 women in Britain undergo IVF or a related technique called Intra Cytoplasmic Sperm Injection (ICSI) every year, according to official figures, resulting in about 12,000 births. Italian researchers looked at 15 studies covering 61,677 births, and concluded that the rate of premature birth was 'considerably higher' in mothers who had undergone IVF/ ICSI than in those who had not.
Even after 'age matching' to take into account the fact that women having IVF tend to be older, there was still a big difference.
On a like-for-like basis, women who had had IVF were 63 per cent more likely to give birth before 37 weeks than those who had conceived naturally. Some studies indicate that 'sub-fertile' women who have trouble conceiving tend to be biologically predisposed to premature birth.
But in the journal Ultrasound in Obstetrics & Gynaecology, the researchers said that studies indicated "the risk of pre-term birth in IVF/ICSI patients is due to the treatment itself.
Women who have IVF with frozen embryos have a better chance of becoming pregnant, the study also revealed. Scientists say the removal of eggs during fertility treatment could damage the womb's lining.
But if implantation could be delayed after egg collection - as enabled by freezing embryos - then the womb could be given time to heal, leading to a better chance of pregnancy.
However, scientists warn that the potential benefits of freezing embryos are highly dependent on the skills of different IVF clinics.
Dr. Gedis Grudzinskas, a Harley Street fertility expert, said: "If implantation could be delayed by a month or two after egg collection, then problems could be overcome.