To nab an under-2 spot at Hansen Place in Brooklyn's Fort Greene, parents need to apply when the mother is in the first trimester of pregnancy.

WSJ: It's Now a Grind For 2-Year-Olds

To get human retinal cells at the same stage of development, however, would involve taking stem cells from a foetus during the second trimester of pregnancy.

BBC: NEWS | Health | Cell transplants 'restore sight'

Those taking part in the study will be asked to go to the antenatal suite at Singleton Hospital in Swansea once during each trimester of their pregnancy and again three months after their baby is born.

BBC: Pregnant women

"Pregnant women are recommended to get a haemoglobin test at least once in every trimester of their pregnancy but it is inconvenient for a women to walk to the nearest primary healthcare centre that could more than 5km away, " he said.

BBC: Device to fight anaemia in India about to go on sale

They also said states could regulate abortion procedures in the interest of a woman's health or in protecting a potential human life starting at the end of the pregnancy's first trimester.

CNN: Before and after Roe v. Wade

Some states have no time limit, while others allow abortion up to the end of the second trimester, about 27 or 28 weeks into the pregnancy.

CNN: North Dakota governor signs law banning most abortions