But who has done the best business this January? Who has given themselves a mid-season boost? Who should have strengthened further?
BBC Sport's Emma Sanders and Jo Currie take a look at the comings and goings and how each team fared in the January window.
Fran Stenson [loan at Birmingham City ended]
Sabrina D'Angelo [Vittsjo GIK] Undisclosed
Gio Queiroz [loan at Everton ended]
Katherine Kuhl [Nordsjaelland] Undisclosed
Victoria Pelova [Ajax] Undisclosed
Jordan Nobbs - [Aston Villa] £40,000
Mana Iwabuchi [Tottenham Hotspur] Loan
Sanders: "Arsenal's activity in January has been frantic. They had several targets which they sorted early on and did some good business, particularly with the signing of young Danish midfielder Kuhl, but injuries to key players meant they were desperately chasing strikers up to the deadline.
"Their world-record bid for Alessia Russo was ambitious and showed intent but they ultimately missed out and will feel they are missing bodies for the second half of the season."
Jordan Nobbs remains revered at Arsenal despite having ended her 13-year stay with the Gunners to join Aston Villa
Jordan Nobbs [Arsenal] £40,000
Lucy Staniforth [Manchester United] Free
Isobel Goodwin - [Coventry United] Free
Currie: "The biggest signings have been at Villa. Bringing in Nobbs and and Staniforth are big statements after bringing in Daly in the summer. It shows the clout that Villa and Carla Ward now have and which direction the club is going in."
Sanders: "Villa have arguably had the best transfer window in the WSL - they want to finish fifth in the table and have added the tools to help them achieve that."
Lydia Williams [Paris St-Germain] Undisclosed
Zoe Morse [Chicago Red Stars] Undisclosed
Brianna Visalli [Houston Dash] Undisclosed
Dejana Stefanovic [Valerenga] Undisclosed
Guro Bergsvand [Brann] Undisclosed
Sanders: "Under new management, Brighton have understandably been busy and have added depth in almost every department. Williams will bring experience in goal and they have a few more options in midfield. I still think they miss a top goal scorer and it could be the difference between staying up or going down."
Bethany England traded Chelsea for Tottenham after scoring 74 times for the WSL leaders
Maika Hamano [INAC Kobe] Undisclosed
Bethany England [Tottenham Hotspur] Undisclosed
Aniek Nouwen [AC Milan] Loan
Maika Hamano [Hammarby] Loan
Charlotte Wardlaw [Lewes] Loan
Currie: "Chelsea are a team that didn't need to do a lot of business and letting England go, even for a decent amount of money, shows that they have plenty of goals in the squad. If they pick up an injury they could miss her, she is one of those players who gets goals."
Sanders: "Chelsea haven't strengthened in the window and they have allowed a few players to leave including England and Nouwen on loan. With Arsenal and Manchester United making strong signings, it could have been a missed opportunity for the Blues but they are planning some major moves in the summer. They should be fine."
Sara Holmgaard [loan at Fortuna Hjorring ended]
Clare Wheeler [Fortuna Hjorring] Undisclosed
Gio Queiroz [loan at Arsenal ended]
Sanders: "A quiet window for Everton after a very busy summer. Sara Holmgaard was a shrewd addition. They were keen to sign goalkeeper Emily Ramsey on a permanent deal from Manchester United but weren't able to. On the whole, I think Everton have what they need to continue their progression from last season's disappointment."
Ruby Mace (left) faced new club Leicester while playing for Manchester City on 18 January
Ruby Mace [Manchester City] Loan
Georgia Eaton-Collins [Free agent]
Courtney Nevin [Hammarby] Loan
Janina Leitzig [Bayern Munich] Loan
Remy Siemsen [Sydney FC] Undisclosed
Natasha Flint [Celtic] Loan
Connie Scofield [Coventry United] Loan
Georgia Brougham - Contract terminated
Sanders: "It's been a very strong window from Leicester and it gives them a chance of staying up. Mace, on loan from Manchester City, is a real coup and is the quality they need. Bayern Munich goalkeeper Leitzig, also on loan, looks very strong too. The only concern is in the summer they will have several players that make up the core of the squad due to finish loan agreements."
Miri Taylor [Angel City FC] Undisclosed
Fuka Nagono [North Carolina Courage] Undisclosed
Sofie Lundgaard [Fortuna Hjorring] Undisclosed
Gemma Bonner [Racing Louisville] Undisclosed
Natasha Dowie [Reading] Loan
Gilly Flaherty [Retired]
Currie: "I think Liverpool have enough in the squad and enough experience to be safe. I know there is a lot of excitement around Nagono and the signing of Bonner comes at a good time because she knows the club inside out. Particularly with Flaherty retiring - one experienced centre-back going out and one experienced centre-back coming in."
Sanders: "Liverpool left it far too late to sign a striker and had limited options but did secure Reading's Dowie on loan before the deadline, which is a strong addition given the timeframe. They have brought in some younger options in midfield which may need time to settle and Liverpool need players to hit the ground running while they are in a relegation battle."
England youth and Manchester City goalkeeper Khiara Keating has been loaned to Coventry United
Ruby Mace [Leicester City] Loan
Khiara Keating [Coventry United] Loan
Currie: "City brought in a lot of young international players last summer and they talk about exciting prospects but it's been a long time since they've signed someone that you've thought 'wow' - for example when they signed Carli Lloyd. Bunny Shaw is going very well but they lost Ellen White, Keira Walsh and Lucy Bronze.
"I don't think they've replaced with the same quality which is strange when you think about the training centre and facilities, which are up there with the best in the world. You could read into it that Gareth Taylor is happy with the squad the way it is."
Lisa Naalsund [Brann] Undisclosed
Jayde Riviere [AFC Ann Arbor] Undisclosed
Estelle Cascarino [Paris St-Germain] Loan
Jade Moore [Reading] Loan
Grace Clinton [Bristol City] Loan
Lucy Staniforth [Aston Villa] Free
Sanders: "United's new signings are exciting and definitely bring quality and further depth to a strong squad. They will be pleased with their transfer activity but there remains huge concern over several key players whose contracts are near expiration - including top scorer Russo."
Williams scores late winner to send Man Utd top of WSL
Jade Moore [Manchester United] Loan
Sanders: "On-loan United midfielder Moore is a strong addition for Reading's low-budget squad. Kelly Chambers wants a striker and had looked at centre-back options in Scandinavia so they still need to add in the summer. I think they needed to bring in more quality to help their relegation battle."
Bethany England [Chelsea] Undisclosed
Mana Iwabuchi [Arsenal] Loan
Gracie Pearse [Bristol City] Loan
Esther Morgan [Sunderland] Loan
Currie: "England could've walked into pretty much any team in Europe, she wouldn't have been short of offers so the fact Spurs have won that battle to sign her is big. Whether they have the players to provide the service she needs to score goals is another thing. I think they will be more than fine [in the relegation battle] with the signings they've made."
Sanders: "Spurs were very ambitious in the window, bringing in England and Iwabuchi - two players who will immensely improve their attacking threat. Their business has been successful."
PSV striker Amalie Thestrup has voiced her excitement at playing in the WSL for West Ham
Anouk Denton [Louisville Cardinals] Undisclosed
Amber Tysiak [Oud-Heverlee Leiven] Undisclosed
Amalie Thestrup [PSV Vrouwen] Loan
Grace Garrad [Stabaek] Undisclosed
Claudia Walker [Birmingham City] Undisclosed
Alex Hennessey [Free agent]
Thea Kyvag [LSK Kvinner] Undisclosed
Sanders: "West Ham have brought in more options in defence and they managed to get striker Thestrup in on loan on deadline day. They need to share goals around the team after relying heavily on Dagny Brynjarsdottir this season and they need Thestrup to help with that. She struggled during a short stint at Liverpool but hopefully West Ham will have enough."