As for many mothers-to-be, the late stages of pregnancy can be extremely awkward for dolphins, say scientists.Gliding along beneath the ocean, it might seem that these streamlined marine mammals are unaffected by the slight swell of carrying a baby.But a study has revealed that the females\' top swimming speed is almost halved when they are close to giving birth.The findings are published in the Journal of Experimental Biology.They reveal just how much the animals invest in carrying their offspring.Lead researcher Shawn Noren, from the Institute of Marine Science at the University of California Santa Cruz, US, was originally interested in how baby dolphins learned to swim.But while she was diving with the animals in Hawaii, and filming their behaviour, she became fascinated by how the females coped with the physical demands of pregnancy.Dr Noren captured more than 30 hours of footage of two female dolphins swimming. She studied these animals for the period from 10 days before they gave birth, until two years after they had given birth to their calves.During their 12 months of pregnancy, dolphins develop a characteristic \"bump\" in their abdomen. Dr Noren used her footage to measure exactly how this affected the animals\' movement.\"When a pregnant animal is swimming at 1.7 metres per second,\" she explained, \"it has the same drag force acting on it as a non-pregnant dolphin swimming at 3.4 metres per second.\"So the pregnant dolphin can only go half the speed as the non-pregnant dolphin before it gets the same drag force.\"Dr Noren also wanted to know if the animals changed the way they moved in order to compensate for this additional drag.Tracing the animals\' movements, she found that pregnant females reduced the size of the arc through which they swept their tails - the up and down sweeping motion that propels the dolphins through the water.\"Pregnant animals had a 13% reduction in the [size] of this stroke,\" Dr Noren explained. \"This might be because of the way the foetus sits so far back in its body... so the abdomen area is too stretched and taught here [and] it limits their movement.\"This slowed the animals down significantly. Their top speed was restricted to approximately 13km/h (8mph), whereas the maximum swim speed of non-pregnant dolphins is more than 22km/h (14mph).To put that into context, the hunting speed of a mammal-eating killer whale - one of the dolphins\' natural predators - is estimated to be in the range of 14-30km/h (9-19mph).\"So you can see how pregnant animals would be much more vulnerable,\" said Dr Noren.Dr William Sellers, a zoologist from the UK\'s University of Manchester, said he was surprised by the magnitude of the cost to dolphins of carrying a baby.\"It\'s not surprising that being pregnant has costs,\" he said, \"all mammals invest a lot in their offspring, but putting an actual number on it... gives us an idea of what it\'s like to be a dolphin.\"He explained that the extra drag meant that the pregnant animals would need twice as much energy to move around.\"Dolphins are this amazing streamlined shape, and it\'s clear that even a small change in that shape affects that streamlining very badly.\"He added that, in order to develop effective ways to protect dolphins and safeguard their environment, scientists needed an in-depth understanding of their ecology.